The unit weigh of water @ 50 degrees F is 62.4pcf if the volume of the vessel is 3.5 cubic ft?

Assuming you mean change in volume from 50-60 deg Fahrenheit.

At 50degF
Volume of water = 3.5ft^3
Density of water = 62.4lb.ft^-3
Mass of water = density of water X volume of water = 62.4lb.ft^-3 X 3.5ft^3 = 218.4lb.

Conservation of mass means mass of water does not change.

At 60degF
Mass of water = 218.4lb
Density of water = 61lb.ft^-3
Volume of water = mass of water / density of water = 218.4lb / 61lb.ft^-3 = 3.58ft^3 (to2d.p.)

a) Difference in volume = 3.58-3.5 ft^3
b) Percentage change = 3.58 / 3.5

Original volume = 3.5ft^3
Density at 60degF = 61lb.ft^-3
Mass of water = density of water X volume of water = 213.5lb

c) Mass reduction = 218.4 - 213.5 lb