The solution would be like this for this specific problem:
Given: x +y = 390
2x + 4y = 1260
2x+4y=1260 ßdivide by 2
= x+2y=630
x+y=390 ßsubtract
= x+2y-x-y=630-390
y=240
x+240=390
x=390-240=150
So, if they raised $1260 from the concession stand, then they sold 240 sandwiches.