ok so the first step would be to distribute 3 into 3x and -6. So 3 times 3x would be 9x. 3 times -6 is -18. 
the equation now is 
18 = 9x - 18
Now you would have to do inverse operation 
18 = 9x - 18
+18       +18
36 = 9x 
Now you would divide 9x into 36 to isolate the variable
36 / 9x = 4
x = 4