Let's call your number x. Difference between your number and nine is doubled, which means that (x-9)•2 and that is equal to eight more than the number, so (x-9)•2 = x+8.
After this we simply calculate:
(x-9)•2 is 2x-18, so now we have
2x-18 = x + 8, move the x on the right to the left side and move -18 on the left side to the right so it becomes
x = 26