Complete the analogy 52 : 77 :: 46 : ? by working out the rule hidden in the first pair.

Write the number backwards and add the reversal to the original: 52 + 25 = 77. For 46 the reversal is 64, and 46 + 64 = 110. The trap 64 is the reversal by itself, with the addition left undone. 10 is only the total of the digits, 4 + 6. 100 is that digit total squared, a rule that fails the first pair because $7\times 7=49$, not 77.