The area of a trapezoid can be found with the formula \(A=\frac{b_1+b_2}2h\) where b1 and b2 are the bases and h is the height. Let's plug in the values that we know and solve for h.
\(3.9=\frac{4+2.5}2h\)
\(3.9=\frac{6.5}2h\)
\(3.9=3.25h\)
\(\boxed{h=1.2\ cm}\)
