Divide first eqn through by 3: x-3y=1. So x=1+3y.
Substitute for x in second eqn: 6(1+3y)-3y=-24
6+18y-3y=-24
6+15y=-24
15y=-24-6=-30
y=-2
So x=1+(-6)=-5. Check out the result by substituting these values for x and y in both original equations:
-15-(-18)=-15+18=3. Correct!
-30+6=-24. Correct!
Note: the second equation could also have been reduced to 2x-y=-8 (by dividing through by 3) to simplify the arithmetic.