To solve this by substitution, you can take one of the equations and get a variable alone on one side, so:
x + 3y = -12
x = -3y - 12
Next, you can plug -3y-12 in for the x values in the second equation to get:
5(-3y - 12) - 3y = -6
-15y -60 - 3y = -6
-18y = 54
y = -3
And with the y value found, you can plug it back into the first equation to find x.
x + 3(-3) = -12
x - 9 = -12
x = -3
