What will be the force of impact on a rectangular body weighing 1200 lb and moving lengthwise at a constant speed of 3 miles per hour when the body hits a stationary wall at 90 degrees and when it hits the stationary wall at 45 degrees?

You cannot say what the force is unless you know how long the two bodies are in contact. In that case, you can only provide an average force.

A rock will impart a greater force than a ball of rubber of the same mass.