Hi i need to work out the equation of the tangent and norm of the following function.

-(25-x^2)^1/2 where x=4

I'm pretty sure i have messed up but here are my solutions that i got.

y= +/- 3

F'(x)= (-4*9^1/2)/9

norm being 9/(4*9^1/2)

If i am correct up to here my equations should be right. Help is greatly appreciated