This is not the best question I've ever seen. The wording is the problem, and it's not your fault. The melting point of Krypton is about - 160 oC. It means that numerically the melting point of Krypton is 4 times lower than that of mercury.
Krypton = 4*M
M = - 39
Krypton = 4*-39
Krypton = - 156o C
Now you want to make it something like K(1/4) = - 39 which would be fine. You can't use - 1/4. If you do, your answer will come out to be + 156 which is not what you mean. If this was a multiple choice question, your answer would be the second best one.