A [m1] kg block is sliding along a frictionless surface. A s…

A [m1] kg block is sliding along a frictionless surface. A second block with a mass of [m2] kg, sliding at a faster speed of [v2] m/s, collides with the first from behind and sticks to it. The final velocity of the combined blocks is [vf] m/s. What was the initial speed of the slower first block in m/s?