How to make m1 the subject of the formula

[cykachu12]

Homework Statement

an object has a total mass of 75kg and is stationary, it then ejects as a smaller object to the left with a velocity of(+ve direction)7 ms-1(v2) with a mass of m2 , the original object then recoils backwards to the right (-ve direction) at a velocity of -2 ms-1 and a mass of m1 .
calculate the mass of m1

Relevant Equations

m2 + m1 = 75kg
m1*v1+m2*v2=0
momentum before event = momentum after event

Homework Statement: an object has a total mass of 75kg and is stationary, it then ejects as a smaller object to the left with a velocity of(+ve direction)7 ms-1(v2) with a mass of m2 , the original object then recoils backwards to the right (-ve direction) at a velocity of -2 ms-1 and a mass of m1 .
calculate the mass of m1
Homework Equations: m2 + m1 = 75kg
m1*v1+m2*v2=0
momentum before event = momentum after event

the answer is total mass * velocity 2
m1 = ------------------------------
velocity 2 - velocity 1 I don't get how on the 2nd attempt i did (in pencil) how -m1*v2 + m1*v1 = -total mass * v2
goes from a + to a - when I factorise.

 

[fresh_42]

Because $-m \cdot (v-w)= (-m)\cdot v - (-m)\cdot w = -mv -(-mw) = -mv+mw$.