Let us suppose the following things,

Mass of smaller ball, m₁ = 2kg

Mass of larger ball, m₂ = 3kg

Combined mass of two balls, m₃ = 5kg

Initial Velocity of Smaller Ball, u₁ = 36 km/h = 10m/s

Initial Velocity of Larger Ball, u₂ = 0 m/s

Final Velocity of both balls combined, v = (?)

By, Law of Conservation Of Momentum, (For this case)

m₁u₁ + m₂u₂ = m₃v

∴2 * 10 + 3 * 0 = 5 * v

∴20 + 0 = 5v

∴5v = 20

∴**v = 4 m/s**

Thus, the final velocity of both the balls moving as a single mass should be 4 m/s.

Now, let us suppose the following,

Initial combined velocity, v₁ = 10 m/s (Larger ball was at rest)

Final combined velocity, v₂ = 4m/s

Combined Mass, m 5kg

Initial Kinetic Energy, K₁

Final Kinetic Energy, K₂

Change in Kinetic Energy, ΔK = K₂ - K₁

= 1/2 mv₂² - 1/2 mv₁²

= 1/2 m (v₂² - v₁²)

= 1/2 * 5 (4² - 10²)

= 5/2 * (16 - 100)

= 5/2 * (-84)

= 5 * (-42)

∴**ΔK = -210 J**

Thus change in Kinetic Energy is 210 J.