First, you'll want to identify the given in the problem.
It costs 100 dollars to purchase all of the shirts and 10 dollars to print the designs on the shirts. The result being a 110 dolar investment prior to his sales. In addition, each shirt is sold for 4 dollars and this becomes the slope of the equation.
Using this info the following equation can be used to find Eric's profit:
P=4x-110
With 4 being the slope.
Overall, after selling the 20 shirts by plugging 20 in for x we get 80-110. So his profit is -30 dollars.
Hope this helps :)
H means multiplication
1/2 x (a+b) x h
h is height
