Solve for substitution:

x = 3y - 7
3x + y = -1

Plug (3y-7) into the second equation as x.

3(3y-7)+y=-1

Now solve it like a regular equation!
First, distribute the 3 into (3y-7)

9y-21+y=-1

Combine the like terms

10y-21=-1

Now, add 21 to both sides of the equation.

10y=20

Finally, divide each side by 10.

y=2

Plug 2 into the first equation as y.

x=3(2)-7
x=6-7

x=-1

To check your answer, plug x and y into the second equation.

3(-1)+2=-1
-3+2=-1
-1=-1

x=-1
y=2

Hope this helped! :)