Solve this system of equations using substitution (show your work).
x + 3 = 7
3x + y.= 7
First, solve the first equation for x:
x = 7 - 3
x = 4
Now, substitute x = 4 into the second equation:
3(4) + y = 7
12 + y = 7
y = 7 - 12
y = -5
Therefore, the solution to the system of equations is x = 4 and y = -5.