Let "x" represent the cost of one pizza and "y" represent the cost of one bread stick order.
Casey: ordered 3 pizzas and 2 orders of bread sticks for a total of $42
You can express the total cost of her order as an equation

Rachel ordered 2 pizzas and 1 order of bread sticks for a total of $26, the equation that expresses her total cost is

With this we have established an equation system with two variables.
Rewrite the second equation in terms of y

Replace the result in the first equation

Solve for x

x=$10 now that we know the value of x, we can calculate the value of y as:

y=$6
This means that each order of bread sticks costs $6