First you substitute 3x-30 into y of 2nd equation.
Step 1: 7(3x-30)-6=3x
Step 2: 21x-210-6=3x
Step 3: 21x-216=3x
Step 4: +216 +216
Step 5: 21x=3x+216
Step 6: -3x -3x
Step 7: 18x=216
Step 8: 18x/18x= 216/18
Step 9: x=12
Now plug in 12 for x in the 2nd equation.
Step 1: 3(12)-30=y
Step 2: 36-30=y
Step 3: 6=y
x=12 & y=6