Hey there! Hello!
For the first problem, all you need to do is plug in the given values into your function, m(x) = 13.50x.
m(300) = 13.50(300)
m(300) = 4,050
m(352) = 13.50(352)
m(352) = 4,752
You can solve the third on your own referencing the work I did for the first two above.
For the second problem, if you're not able to using graphing software, you can just solve the given function c(x) = 10x+50 for one shirt, two shirts, three shirts, etc. Then, you can graph these points and extend the line that you've started to draw out to represent the linear function.
For the third problem, you would need to solve both of the equations with 400 plugged in for x.
m(400) = 13.50(400)
m(400) = 5,400
c(400) = 10(400)+50
c(400) = 4,050
Then, you subtract one of these from the other to get 1,350. That number divided by two will be your answer for the third problem.
Hope this helped you out! :-)