Hello,

When differentiating a function, you can't just put the values... especially here, where it's a nonsense to put the values of y in f'(x)

The equation of a tangent where x=a is :

So calculate f'(x) :

(I'll let you do the calculation... just tell if you can't).

-->

, and not , because a square root is always positive. So -sqrt(25-4²)<0