Triangle KMJ is right triangle with legs KM and MJ and hypotenuse KJ.
By the Pythagorean theorem,
[tex]KJ^2=KM^2+MJ^2.[/tex]
Substitute
KM=6, MJ=3, then
[tex]KJ^2=6^2+3^2,\\ \\KJ^2=36+9=45,\\ \\KJ=\sqrt{45}=3\sqrt{5}.[/tex]
Answer: correct choice is D
