Answer:
sweatshirts cost $42
t-shirts cost $18
Explanation:
9x+3y=288
x+6y=270
substitute 270-6y for x in the first problem
9(270-6y) +3y=288
2430-54y+3y=288
2430-51y=288
51y=-2142
y=42
then plug y=42 into the second equation
x+6(42)=270
x+252=270
x=18