Calculate the average speed between seconds 1 and 4 PLS HELP ME

What is created when solids,liquids,an gases mix with one another

Air is to be preheated by hot exhaust gases in a cross-flow heat exchanger before it enters the furnace. air enters the heat exchanger at 95 kpa and 20°c at a rate of 0.6 m^3/s. the combustion gases (cp = 1.10 kj/kg°c) enter at 160°c at a rate of 0.95 kg/s and leave at 95°c. determine the rate of heat transfer to the air and its outlet temperature.

An object dropped from cliff falls with a constant acceleration of 10 metre per second square find its speed of two seconds after it was dropped