The given function is

Where x is minutes.
(a) To find the number of words typed in 5 minutes, we just need to replace the variable for 5 and solve

Therefore, there are typed 350 words in 5 minutes.
(b) We do the same process for 8 minutes.

Therefore, there are typed 560 words in 8 minutes.
(c) To find the type for 280 words, now we replace the other variable w(x), and solve for x

We divide the equation by 70

Therefore, 280 words take 4 minutes.