Solve for mass of a second object in a momentum question

[AtomicBoom]

Homework Statement

Two objects collide head-on, and are stuck together after collision. Find mass of second object.
Givens:
m1 = 120 kg
v1 = 12 m/s(N)
m2 = ?
v2 =18 m/s (S)
v' = 4.0 m/s (S)

Homework Equations

m1v1 + m2v2 = m1v1' + m2v2'
v'= (m1v1 + m2v2) / (m1+m2)

The Attempt at a Solution

-inelastic collision
-combined mass after collision
-therefore, final velocity will be the same
-I'm not sure on how to rearrange the equation so only one m2 is present in the equation

v'= (m1v1 + m2v2) / (m1+m2)
(4) = ((120)(12) + m2(18)) / (120 + m2)
4(120 + m2) = 1440 + m2(18)
480 + 4m2 = 1440 + m2(18)
480 - 1440 = 18m2 - 4m2
-960 = 14m2
m2 = 68.6 kg

The actual answer is supposed to be 140 kg.

(sorry if this looks confusing, I'm new to this website and haven't quite figured it out yet.)
Thanks

 

[haruspex]

You need to be careful with signs. Each velocity quoted has a stated direction, N or S. Pick whether N or S should be the positive direction and express the velocities accordingly.

 

[AtomicBoom]

You need to be careful with signs. Each velocity quoted has a stated direction, N or S. Pick whether N or S should be the positive direction and express the velocities accordingly.

Oh right, okay.
When I make N positive and S negative and solve it, I get around the right answer.

Thank You!