Which 2-digit number is equal to the sum of its first digit plus the square of its second digit?

Let, the two-digit number = xy

It is given that, xy = x + y²

Then, number which obey the rule is 89.

Check: 8 + 9² = 8 + 81 = 89 [ Itself ]

In short, Your Answer would be: 89

Hope this helps!