Yeah, I tried to do it the normal way and I got to 5293750 = (3x^2 + 250) (2x - 25), which gets me nowhere as I can't factor 3x^2 + 250 without using imaginary numbers, which would be stupid to do as there is no way to undo it with the rest of the equation. @_@
For equations more complicated than a polynomial in x^2, using Excel is a decent way to find a solution.
Fill the first few entries in column A with numbers 1, 2, 3, etc. Highlight all of the entries you've filled, and click and drag the bottom-right corner of the box down the column until you get to box 100.
Then, in another column (say Column B), you type in box 1:
=1.2*(A1)^3 - 15*(A1)^2 + 100*A1 - 140
And press 'enter'. Then, select the cell, drag the bottom right corner down the column as before until you get to the 100th row.
That then gives you the entire list of levels and equivalent experience point total.
EDIT: In fact, if you want to see how much additional experience you need for each level, you can then write in the
second box of a third column (Column C):
=B2-B1
Press enter, repeat the dragging thing.