We have two equations and two unknowns.
y = 6x+7
3x - y = 2
Since the first equation is solved for y, we can substitute the right hand side in for y on the second equation
3x - y =2
3x - (6x+7) = 2
Distribute the minus sign outside of the parentheses
3x -6x -7 =2
Combine like terms
-3x -7 =2
Add 7 to each side
-3x-7+7 = 2+7
-3x = 9
Divide each side by -3
-3x/3 = 9/-3
x =-3
Now we need to find y
Substitute into the first equation
y = 6x+7
y = 6(-3) +7
=-18+7
=-11
The solution is (-3, -11)