square of first digit+square of second digit and reversing the digit we get answer

23 = 31

(2*2)+(3*3) = 13 and reversing the digit 31

similarly

34 = 52

(3*3)+(4*4) = 25 and reversing the digit 25

56 = 16

(5*5)+(6*6) = 61 and reversing the digit 16

so

78 = **311**

(7*7)+(8*8) =113 and reversing the digit **311**