ax-b=c, for x

a=h(a+b)/2, for h

ax-b = c, for x

Add b to both sides

x = b + c

Divide both sides by a

x = ( b + c) / a

Multiply both sides by 2

2a = h (a + b)

Divide both sides by (a + b)

2a / ( a + b) = h