10 POINTS. Find all critical values for the function f(r) = (2r)/(7r^2 +8)

Let a = 2r, so a' = 2
Let b = 7r^2 + 8, so b' = 14r
So f = a/b, so we use the Quotient Rule:
f' = (a'b - ab') / (b^2)
f' = (2(7r^2 + 8) - 2r(14r)) / ((7r^2 + 8)^2)
f' = (14r^2 + 16 - 28r^2) / (49r^4 + 112r^2 + 64)
f' = (16 - 14r^2) / (49r^4 + 112r^2 + 64)
Set that to zero:
(16 - 14r^2) / (49r^4 + 112r^2 + 64) = 0
16 - 14r^2 = 0
14r^2 = 16
r^2 = 16/14
r^2 = 8/7
r = +/- sqrt(8/7)
r = +/- 2*sqrt(2/7)