Please help, I don't understand this question.
2. Suppose that y varies directly with x and inversely with z, and y = 18 when x = 15 and z = 5. Write the equation that models the relationship. Then find y when x = 21 and z = 7. (1 point)

y = 5z /x; 5/ 3
y = 5x/z; 15
y = 6x/z; 18
y = 6z/x; 2

Question

Please help, I don't understand this question. 
2.   Suppose that y varies directly with x and inversely with z, and y = 18 when x = 15 and z = 5. Write the equation that models the relationship. Then find y when x = 21 and z = 7.  (1 point)

y = 5z /x; 5/ 3 
y = 5x/z; 15 
y = 6x/z; 18 
y = 6z/x; 2

hero · Answer

Hint:
yz = kx

hero · Answer

1. Plug the 1st set of given values into the formula
2. Solve for k
3. Plug the second set of given values into the formula including the value for k 
4. Solve for y

hero · Answer

`1.` (18)(5) = 15k
             90 = 15k
        90/15 = k

`2.`  6 = k
`3.`  7y = (6)(21)
         7y = 126
           y = 126/7

`4.` y = 18

anonymous · Answer

Thank you so much.  I just got back from class.