If the average of three numbers is V if one number is Z and another is y what is the remaining number?

The formula for an average of three numbers is (x+y+z) / 3. So if V equals the average, then we find the formula

V = (x+y+z) / 3

We are now looking for x. We can divide each of the variables in the parentheses by 3, making the formula

V = x/3 + y/3 + z/3

So now bring the unwanted variables to the same side as V.

V - y/3 - z/3 = x/3

Now multiply both sides by 3 to find x.

3V - y - z = x

x = 3V - y - z