The sum of y and z decreased by the difference of m and n

The sum of y and z decreased by the difference of m and n.

Take this algebraic expression in 3 parts:

1. The sum of y and z means we add z to y: y + z
2. The difference of m and n means we subtract n from m: m - n
3. The phrase decreased by means we subtract the quantity (m - n) from the sum (y + z)

(y + z) - (m - n)