A trapezoid has an area of 123.265 square miles. One base is 17.8 miles long. The height measures 8.9 miles. What is the

The area, A, of a trapezoid is given by :

A =  height *  ( sum of the base lengths) / 2

So we have

123.265 = 8.9 * ( 17.8  + missing base)  / 2

Multiply both sides by 2

247.25  =  8.9 * (17. 8 + missing base ) 

Divide both sides by 8.9

27.78   =  17.8 + missing base   

Subtract 17.8  from both sides

≈ 9.98 miles  = missing base