Double the first equation: -10x+16y=42. Second equation probably should be 10x+3y=15, so add this to the other equation: 16y+3y=42+15, 19y=57, y=57/19=3. Note that the x term has been eliminated, allowing y to be calculated first.
-5x+8y=21 (given equation),
-5x+24=21, 5x=24-21=3, x=⅗.
SOLUTION: x=⅗, y=3.