so the formula for the area of a trapezium is
area = (a + b)/2 h
a is the short side
b is the long side
h is the height
so let's plug in the values we already know: area, long base, short base. Here's how the formula looks now...
6550 = (150 + 85)/2 h
so, we need to find 'h'
use algebra to find it.
6550 = (235/2) h
6550 = 117.5 h
6550/117.5 = h
h = 55.74 (approx)