I don’t see a picture but to length x width x height :)
Two angles are said to be supplementary if their sum is 180 degrees. So, if x and y are supplementary angles, the formula is

We know that one of the angles (say x) is 98. So, the equation updates to

If we want to solve for the other angle, we can simply subtract 98 from both sides to get

When it says varies directly, it means that y and x are going to be on opposite sides of the equals sign and that there is a constant, k,
y=kx
Plug in (5,100)
100=k(5)
K=20