We know that y varies directly with x to the power of 3, which means that we can express y as:

where k is the constant of proportionality. To determine the value of k we plug y=6 and x=2 in the expression above and solve the resulting expression:

Hence the value of k is 3/4 and the expression for y is:

Plugging x=4 we have that:

Therefore, when x=4 the value of y is 48.