Side one will be x, and side two will be y.
x = 2y
x + y + 8 = 26
Subtract 8
x + y = 18
Remember that x = 2y
The only value that would make this expression true, is 6.
12 + 6 = 18
Now we can plug these values in to x + y + 8 = 26
12 + 6 + 8 = 26
That's it.