The area of a trapezoid is
A = (1/2) (h) (b1 + b2).
( 'h' is the height of the trapezoid. 'b1' and 'b2' are its two bases.)
You want the equation solved for one base.
A = (1/2) (h) (b1 + b2).
Multiply each side by 2 : 2A = h(b1 + b2)
Divide each side by 'h' : 2A/h = b1 + b2
Subtract b1 from each side: 2A/h - b1 = b2