The area of a triangle is given by the formula

, where B is the base and h is the height. We can rearrange this formula to solve for B.

.
We plug in the given area, 640 square millimeters, and the given height, 32 millimeters.

.
40 mm is our final answer.
Work is provided in the image attached.
Although you did not include units, I just used centimeters.
However, if there were no units, then just put your answer.
If you do not understand my work,
please ask me in the comments below.
My only guess is a^2 + b^2 = c^2