First, let's start with the obvious errors, 2 is not equal to 5, 3 is not equal to 10, and 4 is not equal to 16. If those things were true, then we know that 5=2 by the reflexive property.

I don't believe that is the answer that is expected, and I don't like redefining a basic piece of notation.

So l we need to fix this problem a little. I going to assume that they are trying to represent some relation by using the equals sign. I'm going to rewrite this:

x | y |

2 | 5 |

3 | 10 |

4 | 17 |

5 | ?? |

y = 26

However, there are many functions that can fit just three points.

Here is a parabola, that fits these points. Using this function f(5) = 23.

And then there is this circle: . There are two answers here: 24 or 59.

So so far we have answers of 2, 23, 24, 26, and 59. Are there more? Probably. I bet some sort of periodic function could fit those three points. Comment if you think you know of another answer.