Product of two numbers is positive if both are positive or both are negative. Mostly your velocity u is negative and thus 2 - 3e must also be negative.
A particle of mass is moving with speed in a straight line on a smooth horizontal table. The particle collides with a particle of mass moving with speed in the opposite direction to .
a) Show that the speed of Q after the collisions is .
As a result of the collision, the direction of motion of is reversed.
(b) Find the range of possible values of .
[It's part b that we are discussing, which a follow on question]