z varies jointly as x and y. If z=3 when x=3 and y=15, find z when x=6 and y=9

z varies jointly as x and y. If z=3 when x=3 and y=15, find z when x=6 and y=9

Varies jointly means there exists a constant k such that:
z = kxy

We're given z = 3 when x = 3 and y = 15, so we have:
3 = 15 * 3 * k
3 = 45k

Using our equation solver, we see that:
k = 1/15

So our joint variation equation is:
z = xy/15

Then we're asked to find z when x = 6 and y = 9
z = 6 * 9 / 15
z = 54/15
z = 18/5