x-12=30
In the problem it states a number (x) decreased (-) by 12. So with that information you're equation should look like x-12. The next part of the problem says that it is equal (=) to 30 So now you're equation should be set up as x-12=30. To find "x" you must isolae "x". To do that you would add 12 from the left hand side leaving x and then you would also add 12 to 30. And with that you're left with what x is.
x=42
Hope it helps!