Diagonals that bisect each other are perpendicular and form a right angle.
AEB is a right triangle.
We can apply the Pythagorean theorem:
c^2 = a^2+b^2
Where c is the hypotenuse (longest side) and a and b the other 2 legs.
Replacing:
AB^2 = AE^2+BE^2
AB^2 = 5^2+12^2
AB^2 = 25+144
AB^2 = 169
AB=√169 = 13
Lenght of AB= 13