The 100 kg block in figure takes 7.00s to reach the floor after being released from rest. What is the mass of the block on the left?

Answer 1

No figure was provided. It sounds like an "Atwood machine" problem, with two masses connected by a pulley.

The acceleration of such a device is
a = g*(M1-M2)/(M1+M2) if friction and pulley mass can be ignored.

See http://hyperphysics.phy-astr.gsu.edu/hbase/Atwd.html
for more about this kind of problem