Draw 2 parallel lines (matters not how far apart they are), AB = 25 cm on top of DC = 37 cm so that the vertices from top left read, clockwise A, B, C and D. Let P be the mid-point of BD and Q the mid-point of AC. Draw a line thru P & Q to intersect AD at R and BC at S. R is mid-point of AD, and S, of BC.
Now look at triangle ABD. RP = 0.5 x AB = 0.5 x 25 cm (triangles DRP ||| DAB). Similarly, looking at triangle ADC, RQ = 0.5 x 37cm. .: PQ = RQ - RP = 0.5(37 cm - 25 cm) = 6 cm.