We will have to use the formula for the area of a trapezoid on this problem, which is ([tex] b_1 [/tex] and [tex] b_2 [/tex] are the bases of the trapezoid and [tex] h [/tex] is the height):
[tex] \dfrac{b_1 + b_2}{2} \cdot h [/tex]
All we need to do is substitute the information we are given into our formula:
[tex] \dfrac{21 + b_2}{2} \cdot 4 = 68 [/tex]
Now, let's just simplify the equation:
[tex] \dfrac{21 + b_2}{2} \cdot 4 = 68 [/tex]
[tex] \dfrac{21 + b_2}{2} = 17 [/tex]
[tex] 21 + b_2 = 34 [/tex]
[tex] b_2 = 13 [/tex]
The missing base has a length of 13 units.
