Assuming it's a linear function, you can solve this by doing the following;
1.) Fine the slope - y2 - y1 / x2 - x1
26 - 15 / 3 - 2
11 / 1
The slope is 11
2.) Standard form for a linear equation - y = mx+b
26 = 11(3) + b
26 = 33 + b
-7 = b
The final answer (in standard form) is y = 11x - 7
If you need it in any other form just comment and I'll help you out
