For a trapezoid, the following formula relates the area A, the two bases a and b, and the height h.

A=(a+b)2hA=(a+b)2h

Solve for a.

a=2Ah+ba=2Ah+b

a=2A−bha=2A−bh

a=2Ah−ba=2Ah−b

a=Ah2−b

2 answers

h(a+b)/2
not times 2
average base times height, common sense
A = h (a+b)/2
2 A/h = a + b

a = 2A/h - b