The formula for the area of a trapezoid is A=frach(b1+b2)2A = \\frac{h(b_1+b_2)}{2}A=frach(b1+b2)2. Solve for b1b_1b1.
b1=frac2Ah−b2b_1 = \\frac{2A}{h} - b_2b1=frac2Ah−b2
b1=frac2A−b2hb_1 = \\frac{2A - b_2}{h}b1=frac2A−b2h
b1=frac2Ah+b2b_1 = \\frac{2A}{h+b_2}b1=frac2Ah+b2
b1=2A−h−b2b_1 = 2A - h - b_2b1=2A−h−b2