EF Is A Median Of Isosceles Trapezoid ABCD. The Length Of AB Is 12, And The Length Of CD Is 18. The Length Of BF Is 5. What Is The Length Of DE?

DE is also 5.
Each of the segments BF, FC, DE, EA will be 5. The fact that the trapezoid is isosceles means DA = BC. The fact that EF is a median means DE = EA and BF = FC. Of course, DE + EA = DA and BF + FC = BC.