What is the simplified answer of 5 -5/2 x 5 9/2

Acircle has a radius of 5/6 units and is centered at (3.6, 7.8) write the equation of this circle

The diagonals of a quadrilaretral intersect at (-1,4). one of the sides of the quadrilateral is bounded by (2,7) and (-3,5) determine the coordinates of the other side in order for the quadrilaretral to be a square.

Asap! i'm not good math and i'm lost.