Use k = y

Question

x
and y = kx
Suppose y varies directly
as x, and y=4 when x=5.
Find x when y=6.

Answer 1

I will assume your question is simply:

Suppose y varies directly as x,
and y=4 when x=5.
Find x when y=6.

y = kx
when y=4, x=5
4 = 5k
k = 4/5

so y = 4/5 x
when y = 6
6 = 4/5 x
30 = 4x
x = 30/4 = 15/2