I have to finish an assignment in the next couple of days. One of the questions asks to show how the pitch of the blade must change along the length so that the blade does not stall. I have no idea what I have to do or how, can someone help me please?
The question should really read, "how the pitch must change along the length to keep lift distribution reasonably constant while respecting stall considerations" or something like that. Rotor blades twist nose down going outboard, like propellers but with less twist, to account for the higher velocity toward the tip, so that the slower moving root end is doing a decent amount of work making lift.
This tends to make the root end operate closer to stall AOA than the tips. Whether it actually stalls or not is a more complex function of blade forward velocity (rpm) and vertical circulation velocity down through the rotor disc, whether forward airspeed is involved, whether you want to optimize for hovering or forward flight, and so on.
You're going to have to do some reading. I found an ASE post from a couple of years ago by @Koyovis, relating to a question about blade twist, that links to a great information source, a book at Google Books that you can read online, called Helicopter Aerodynamics Vol. 1 by Ray Prouty, who wrote a column for Rotor & Wing for many years. You should be able to assemble the information you need from that.
As referenced above I'm assuming that it is a (helicopter?) rotor blade. The way this is normally dealt with is blade twist, such that there is a high angle of attack (AoA) close to the root, which progressively reduces out along the span such that is there AoA is perhaps 10-20 deg less at the tips. Some more modern machines with composite blades also incorporate variation in camber and chord thickness along the blade, for the sane purpose.
The thing is however, none of these tricks actually prevent part of the blade from stalling. The inner 1/3 (roughly) of the disc has tangential airspeed, from the rotation,that is still low it is nearly useless. The middle section and the tips do most of the work. I think you'll be wise to ignore the inner part, assume zero effective forward airspeed, no variation in camber or thickness (ie take it to be a uniform chord blade type) and do some math on the pitch angle changes required along the blade span, to compensate for increasing airspeed as radius increases.