Can somebody help me with this and explain me how to resolve it

A=1/2(b1+b2) for b1

I think it means you have to change the subject of the formula to b1. So:

A = 1/2(b1 + b2)

That's the same as saying:

A = (b1 + b2)/2

So:

Cross multiply the 2 to get:

2A = b1 + b2

Here you simply take b2 off both sides to solve for b1, so your answer is:

b1 = 2A - b2

Hope that helps 😄