I took moments from a different points to the answer and I am incorrect? A level math

Must admit, its not clear what youve done, but if youve taken moments about B, then you have
clockwise: normal reaction at A
anti clockwise: weight and friction
You seem to have missed out the nomral reaction moment and got the wrong direction for friction moment?

I have take moments about A

I assumed that the force at B = Frictions

The force at B I have written as Rb
so if I take moments abot A it should be:

Rb x 4sin (alpha) = 40g x cos(alpha)

Hmm, that assumption isnt right. You dont know the angle or magnitude. I guess youve assumed its horziontal which visually it clearly isnt.

ahhhhhhhhh i understand. Thank you !

I have take moments about A

I assumed that the force at B = Frictions

The force at B I have written as Rb
so if I take moments abot A it should be:

Rb x 4sin (alpha) = 40g x cos(alpha)

Wait, why would you take moments about A if you're trying to find the normal reaction at A? That would get rid of the reaction force from your equation, so you wouldn't be able to calculate it anyway.

Thank you @toxicgamage56 and @davros, I think this is why I often get confused for moments, this will help me massively!

I dont know where ive gone wrong here too, the answer is meant to be tan =2. In the answer they took moments about A

Looks like your sin and cos are wrong on the right hand side. Obviously the weight should be the "same trig" as Ra and Fr should be the other.