Let k be Kaden's current age and s Sally's current age.
Right now, Kaden is 18 years older than Sally, so
k = s + 18
In 5 years, Kaden will be 3 times older than Sally, so that
k + 5 = 3 (s + 5)
Substitute the first equation into the second one to solve for s :
(s + 18) + 5 = 3 (s + 5)
s + 18 + 5 = 3s + 15
s + 23 = 3s + 15
3s - s = 23 - 15
2s = 8
s = 4