First thing you need to do is find your k in y = kx
So to do that insert 18 for x and 6 for y and then solve for k
y = kx
6 = k18
6/18 = k18 /18
1 / 3 = k
k = 1 /3
So now that we know what k is we can find x when y = 10. Once again insert the numbers we know. We know k = 1/3 and y = 10 and we need to find x
y = kx
10 = (1/3)x
3* 10 = 3*(1/3)x
30 = x
x = 30