So just your normal equation.
We need to isolate x alone.
First lets write out our equation
3 +7x = 12
Lets do this step by step.
first subtract 3 from both sides.
3 + 7x = 12
-3 = -3 it cancels out on the left side and we solve on the right side.
*************
7x = 9
second we divide both sides by 7
7x = 9
÷7 = 7 we got the x alone and we just write 9/7 as our answer.
x = 9/7
Or as a decimal.
9/7 could be 1.28571428571429
Either will work.
x = 9/7
or
x = 1.28571428571429
Have a nice day. :)