Method 1: By substitution
Step 1: In the equation 3x + 2 = y when you see a y replace it with -1
3x + 2 = -1
Step 2: Combine like terms by subtracting 2 to both sides
3x + (2-2) = -1 - 2
3x = -3
Step 3: Isolate x by dividing 3 to both sides
x = -1
Method 2: By looking at the graph
To solve this way you see where the line on the graph has y = -1 and look for its complimentary x
If you look at the graph attached below you can see that at y = -1 the x also equals -1
x = -1
Hope this helped!