Question 84299
This is much easier than it looks.  Joint variation means that the variable varies as the PRODUCT of two or more variables.  By the way, was this supposed to be "Z varies jointly as a and b"???  

Using k as the constant of proportionality, write an equation that expresses: Z varies jointly as a and b. 
Z=k*(ab)
Z=kab


R^2 at SCC