nobody can help?

I feel like I'm going crazy or something. I cannot find a problem, I have calculated the dw/ds like 5 times and set it to zero, and I get, 12, -12. The correct answer should be 6.

Can anyone find the derivative of s=w(144-w^2)^1/2? Set it equal to zero, what do you get?

I am wondering if my calculator is just unable to find the 6?