Please help this is the last problem

Evaluate the expression for the given value of the variable 2×(c2-5) for c=4

Look at the figure below: triangle abc is a right triangle with angle abc equal to 90 degrees. the length of ac is 5 units and the length of ab is 4 units. d is a point above c. triangle adc is a right triangle with angle dac equal to 90 degrees and dc parallel to ab. what is the length, in units, of segment cd?

Which term describes the point where the perpendicular bisectors of the three sides of a triangle intersect?