How do I substitute y=x+3 and y=2x+12

(-9,-6)

Step-by-step explanation:

Since both equations are equal to y, they are equal to each other, so substitute for y by setting the equation as x+3=2x+12 and solve for x.

x+3=2x+12, subtract x from both sides and 12 from both sides.

-9=x

Then substitute - 9 for x in either equation to solve for y

y=-9+3

y=-6