Having trouble findin the Arc length of this curve.

x=(1/3)(sqr*(y))(y-3) 4<y<25

I found the first derivative: (sqr*(y)/2 - 1/2sqr*(y)) Is that right?

Then put it into the arclength forumule to set up the integral

sqr*(1+ (sqr*(y)/2 - 1/2sqr*(y))^2)

Now sure how to simplify the terms underneath.