Calculate function value at given point with app?

Questions
1

Ok so I want for example calculate something as easy as f(x) = x^3 + 3x for x = 5 in the app. But how do I do it?
I have tried to type things such as; f(x) = x^3 + 3x , x = 5 but it ends up being wrong all the time. How do I type it correctly?

Thanks.

2

to calculate the derivative of a function at a given point

example

f[x_]:= 2x^2 +5x -Sin[x]
f'[x]/.x->2

or

f'[2]

pay attention to the definition of the function

in WA . simply type "derivative of " (your function) " where " x= value