Question 895015
f(k) means to replace every x you see in the function with k.

f(k) = 3x^2 + 4x + 5

f(k) = 3k^2 + 4k + 5

Did you follow?

For more practice, do this one on your own.

Find f(apple) = 3x^2 + 4x + 5.

Replace every x with apple and you're done.