A piece of cardboard is 3 cms longer than it is tall. The area of the piece of cardboard is 70 cms. How many cms tall is the piece of cardboard?

W+3=L and Area=W*L=70

So,
W*L=70, or
W*(W+3)=70
W^2+3W=70
W^2+3W-70=0
W=-3/2 + SQRT((3/2)^2+70) via quadratic formula
W=7

then,
W+3=L
7+3=L
L=10