The formula of an area of a trapezoid:
[tex]A=\dfrac{base_1+base_2}{2}\cdot height[/tex]
We have:
[tex]A=87\ m^2\\\\base_1=13\ m\\\\height=4\ m[/tex]
Substitute:
[tex]\dfrac{13+base_2}{2}\cdot4=87\\\\(13+base_2)\cdot2=87\qquad|\text{use distributive property}\\\\(13)(2)+(base_2)(2)=87\\\\26+2base_2=87\qquad|\text{subtract 26 from both sides}\\\\2base_2=61\qquad|\text{divide both sides by 2}\\\\base_2=30.5\ m[/tex]
Answer: 30.5 m
