In any trapezoid, the area is given by
[tex] A = \dfrac{(B+b)h}{2} [/tex]
where B and b are the two bases, and h is the height. You're given all these elements already, so you only need to plug the values in:
[tex] A = \dfrac{(3+11)\cdot 8}{2} = \dfrac{14\cdot 8}{2} = 7\cdot 8 = 56 [/tex]
