Subtract the first equation from twice the second.
2(4x +5y) -(8x -3y) = 2(-9) -(21)
8x +10y -8x +3y = -18 -21
13y = -39
y = -3
Then we can find x by substitution into the first equation.
8x -3(-3) = 21
8x = 12 . . . . . . . subtract 9
x = 12/8 = 3/2
The solution is (x, y) = (3/2, -3)