is x = | y - z | ?
x = y - z
y > z
Since absolute value can't be negative, for x to be equal to |y - z|, x must be greater than or equal to 0.
(1) If y > z, then x is positive and x = |y - z| = y - z. However, if y < z, then x is negative and cannot be equal to the absolute value. Insufficient.
(2) This tells you nothing about x. Insufficient.
Together: Since y > z, we know that y - z > 0. Therefore |y - z| = y - z, and we know that x = y - z. Sufficient. (C).