-(m-2n)+(3a+b)=

(x-5)-(7-x)+(9-x)=

= - m + 2n + 3a + b = 3a + 2n + b - m= х - 5 - 7 + х + 9 - х = (х + х - х) + (9 - 7 - 5) = х + (-3) = х - 3