To solve this, you must set up a system of equations.
We'll say white shirts = w
Yellow = y.
w+y=18 (a total of 18 shirts sold)
9.95w + 11.5y = 200.8.
You could then solve this by changing the first equation to y = 18 - w and plugging this in for y in the second equation to look something like this:
9.95w + 11.5(18-w) = 200.8
Solve for w, and then you can solve for y.
Hope that helped! If you need any more help I'm available for online tutoring.