the diagonals of a rectangle are congruent
meaning AC and BD are congruent
c/3 = 4-c
multiply both sides by 3
c = 12 -3c
add 3c to both sides
4c = 12
divide both sides by 4
c = 3
So the diagonals are
AB = c/3 = (3)/3 = 1
BD = 4-c = 4- (3) = 1
Hope this helps