Hi!
To solve for x, you want to isolate x, getting rid of any coefficients it has, and moving all other numbers to the other side.
So, you can subtract 9 from both sides, to get:
4x + 7x + 9 - 9 = 31 - 9
4x + 7x = 22
Now add 4x and 7x.
4x + 7x = 22
11x = 22
And divide both sides by 11 to get rid of the coefficient.
x = 2
Hope this helped!