Answer: b = 6
Work Shown
area = base*height
60 = b*10
b = 60/10
b = 6 is the base.
The side 11 is never used at all. It was put there as a distraction.
Notice the area of a parallelogram formula is the same as the area of a rectangle formula. This is because any rectangle is a parallelogram, but not the other way around. The base and height are always perpendicular to one another.