ANSWER
15.18
Step-by-step explanation
If z varies directly as x, then the relationship between them is,

But z also varies directly has y, so the relationship between z and these two variables is,

We have to find k, knowing that z = 51 when x = 36 and y = 3,

Solving for k,

Thus, the relationship is,

Now, if x = 25 and y = 7, then z is,

Hence, the value of z when x = 25 and y = 7 is 15.18, rounded to the nearest hundredth.