First, draw the 2 quadrilaterals side by side. have them the same basic shape but different sizes.
label the smaller one pqrs and the larger one ghjk (put the letters along the sides instead of at the corners.)
make sure the letters correspond correctly, so for example side p is in the same relative position as side g in the other shape.
now, side p is 3, and side g, which corresponds with p, is 5.
side s = 8, and you have to find its corresponding side, which is k.
so think of proportions: 3 is to 5 as 8 is to k.
3/5 = 8/k. Cross multiply and you get 3k = 40, so k must be 40/3, or 13.333.....
hope this helps!