If x = 4, y = 2 and z = 1 then xz (4y - 2z) is

To solve this problem, you'll want to substitute the variables for the values you are given and follow the order of operations. Let's substitute first, shall we?

xz becomes 4 times 1.
4y becomes 4 times 2.
2z becomes 2 times 1. 
Now that we have everything substituted in, we have our new equation.
4*1(4*2 - 2*1) With the asterisks representing the multiplication sign.
Now, solving those first easy problems as the first step, we get..
4(8-2)
Remember to follow order of operations! Do the problem in the parentheses first, 8-2.
Now we have 4(6).
Assuming the problem is exactly how you wrote it and you are supposed to multiply the two numbers, your answer is 24.