I guess you want to calculate the value of x and y. If so:
Let x= # of pizza orders & the total cost of the pizza order = 2.5.x
Let y = # of chicken sandwich & the total cost of sandwiches order = 3.5.y
x+y = 7
205x + 36.5y = 21.5
Solving this equation gives:
x=3 (pizza) and 4 (sandwishes)