First of all, add 7 to both sides, in order to cancel the -7:
[tex] -7 + 4n +7= 9+7 [/tex]
As we said, the -7 on the left hand side cancels out, while we can add it to the 9 on the right hand side:
[tex] 4n = 16 [/tex]
Now, divide both sides by 4. As before, this is needed to cancel the four on the left hand side and isolate the x, while it is just a normal division between numbers on the right:
[tex] \dfrac{4n}{4} = \dfrac{16}{4} \iff x = 4 [/tex]
