In an isolated system, bicycle 1 and bicycle 2, each with a mass of 30 kg, collide. bicycle 1 was moving to the right at 2 m/s, while bicycle 2 was moving to the left at 4 m/s. what is the magnitude of their combined momentum before the crash?

Momentum is generally defined as mass in motion. Therefore, every mass in motion has momentum. Momentum is related to mass and motion. As the mass and/or speed increases, momentum increases. Additionally, momentum is a vector quantity and depends on the direction of motion.

Mathematically;
Momentum = mass*velocity

In the current case:
m1 = m2 = 30 kg
v1 = 2 m/s
v2 = 4 m/s

If motion to the right is taken positive and that to the left is negative, then momentum of the two bicycles before collision is given by;
Momentum = mass(v1-v2) = 30(2-4) = -60 kg-m/s (that is, the momentum is 60 kg-m/s to the left).