x | 12 | 24 | 26 |
y |  5 | 10 | 15 |

We can assume it is of this form, 
a quadratic function: 

f(x) = Ax² + Bx + C

f(12) = A(12)² + B(12) + C = 5
      =   144A +   12B + C = 5

f(24) = A(24)² + B(24) + C = 10
      =   576A +   24B + C = 10

f(26) = A(26)² + B(26) + C = 15
      =   676A +   26B + C = 15

So we have the system of equations:



Solving that system gives



So one function that fits the data of the table is:



Edwin