Sale $8 off
4 units were purchased at a total cost of $42
The sale off means that $8 dollars were discounted to the price of every shirt, if x represents the original price of the shirts, then x-8 is their price with the discount for each shirt.
For the 4 shirts the total cost can be symbolized as:
4(x-8)=42
Now that is expresed in an equation with one unknown, you can clear the value of x:
Step one, distribute the multiplication:

Next is to let the terms with x in one side of the equation and the integers in the other side:

Now divide both sides of the equation by 4 to get the value of x

Each shirt costs $18.5