Regression in a specific equation

I have values from field tests.

And I need to do the regression fitting in the next equation:

y = ax^n + bx

But I try with linear regression, no linear, etc and not works.

Data for the example:

+-----+---------+
|  X  |    Y    |
+-----+---------+
|  1  |      7  |
|  3  |    255  |
|  5  |   1895  |
|  8  |  12320  |
+-----+---------+