I have a problem with a supposedly easy question...

Find the points on the hyperbola $\displaystyle \frac {x^2} {25} - \frac {y^2} {9} = 1 $ where the slope of the tangent is equal to 1.

i end up with $\displaystyle y' = \frac{9x}{25y}$ where y' becomes 1, and i get another function........

the listed answer should be $\displaystyle ( \frac{-25}{4} , \frac{-9}{4} ) and ( \frac{25}{4} , \frac{9}{4} ) $