i have fopur questions like this but don't undersand them. thanks

Rearrange the formula a=(7/b+1)-4 to make b the subject

a=(7/b+1)-4

a+4 = 7/b + 1

b(a+4) = 7 + b

b a + 4b - b = 7

b (a+3) = 7

b = 7/(a+3)