If x and y vary directly, and x=10 when y=3, what will be the value of x when y=9?

As x and y vary directly then x∝y i. e. x=ky, where k is a constant.

When x=24, y=8 i. e. 24=k*8 and therefore

k = 248 = 3

and relation between x and y is hence x=3y

and when y=9, x=3*9=27.

I hope this is what you're looking for.