The answer should be:
20.875
Hope this helps.
First, expand to make it easier
y=x(x^2+5)(x^2+5)
y=x(x^4+10x^2+25)
y=x^5+10x^3+25x
differentiation
y'=5x^4+30x^2+25 is the derivitive
X<3 or X>1
move constant to the right side and change the sign
Since the square root of 13 is something really weird (3.6055 something) and does not have a clear end or repeats, it is irrational. Solve for the number before assuming anything