A box has a total surface area of $100.$ The length of the box is equal to twice its width, as well as equal to $3$ less than its height. Wh

Did you directly copy it from somewhere else without proofreading over it?

Please do not, because the LaTeX may mess up.

Let length = L, width = W, height = H.

\(2(LW + WH + HL) = 100\\ L = 2W\\ L = H - 3\)

Changing the subjects of the second and third equations,

\(W = \dfrac L2\\ H = 3 + L\)

Substituting these into the first equation:

\(2\left(L \cdot \dfrac L2 + \dfrac L2 \cdot (3 + L) + (3 + L) \cdot L \right) = 100\\ L^2 + 3L(3 + L) = 100\\ 4L^2 + 9L - 100 = 0\)

Solving by quadratic formula gives \(L = 4\)

I believe you can work out the width and the height from here.  

thank you! i got it now. sorry i will proofread next time