Question

If y= ab, then b=
a+b
A line goes under ab. Please help, I have not taken math in many years.

Answer 1

I am assuming you mean

y = ab/(a+b)

then cross-multiply
ay + by = ab
by - ab = -ay, simply moved stuff around
b(y-a) = -ay , factored out the b
b = -ay/(y-a) , divided both sides by y-a