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Are you sure that you copied the question here exactly as it is given in your homework? I ask because the first equation does not contain "y".
If it is correctly copied, then you solve the first equation for "x", and then plug that expression into the "x" in the second equation, like this:
first equation:
3x + 8 = - 14 (subtract 8 from both sides)
3x = -22 (divide both sides by 3)
x = -22/3
second equation (use substitution now, plug -22/3 into "x" here):
y = 6(-22/3) - 6
y = -132/3 - 6 (OR, y = [2 *-22] - 6, either way is correct).
y = -44 - 6
y = -50
That is how you solve by substitution. You solve one of the equations for one of the variables, and then plug that solution into the other equation for that variable.