Question

so

Answer 1

y varies directly with x

means that
y= k*x

y is x times some number k

if they give you an x and y you can find k
in your case, replace y with21 and x with 7
21= 7k
can you find k? (divide both sides by 7)

once you know k, replace k with the number you found in
y= kx
now if they give you a different x, you can multiply it by k (the number) to find y

Answer 2

of course for
what is y when x=0

it really does not matter what k is when x is 0
y = k*0 is easy to figure out