Answer: y = -4
Step-by-step explanation: To substitute 3 for x in the equation, we just need to plug the number 3 whereever we see an x.
y = x - 7
y = 3 - 7
Now we need to solve for y. To do so, we just need to solve everything that is on the opposite side of the equal sign.
y = 3 - 7
y = -4