x = long sleeve
y = short sleeve
x+y = 375
23x + 28.50y = 9862.50
You can solve the equation by a number of different ways including finding the inverse of the equation, using substitution, using elimination, etc.
I personally would solve it by using elimination. And to do this, you would multiply x+y=375 by -23 to get -23x-23y=-8625.
Now you would put the equation above the second equation
You can cancel out the "x's" to get 5.50y=1237.50 You then divide 1237.50 by 5.50 to get 225. So there are 225 short sleeve shirts. Plug this value back into x + 225=375 and you will get 150 long sleeve shirts.