Sequence is a(n) = (n+3)^2 , where n is a natural number, and a(n) is the element of the series.
So x is at position n=3, so x = a(3) = (3+3)^2 = 6^2 = 36
Also, a(k) = (k+3)^2 = 169 => k+3 = sqrt(169)
Neglecting negative value we get:
k+3=13
k =10,
so 10th term will have the value of 169