A = 1/2bh
We want to solve the equation for b, so we want to get the b term by itself
Divide both sides by 1/2:
A/(1/2) = (1/2*b*h)/(1/2)
This causes the 1/2 to cancel on the right hand side.
And now A/(1/2) = 2A
2A = b*h
Next we need to divide both sides by h:
2A/h = (b*h)/h
This causes the h terms to cancel out on the right hand side leaving us with:
2A/h = b
And we have solved for b!