The formula for the area of a trapezoid is a=1/2h(b+b) express b in terms a, h,and b2

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

What do you mean by that?