The statement "y varies jointly as x and z" is equivalent to saying
y= kxz where the k is a constant. Varies jointly means proportional. For example, if the product of x and z doubles, so will y. To find k, just plug in the given values of y, x, and z.
Now that you know k, you can solve for an unknown y when given x and z because k doesn't change numeric value. Plug in the given x and z values.