When integrating an equation we can treat each variable individually. Lets start with 4x^3, when integrating, we raise the power (in this case 3) by +1 and divide the multiple (in this case 4) by the new raised power (in this case 3+1=4).
The integral of 4x^3 is therefore: (4/4)x^4 i.e. x^4
We follow the same process to integrate -3x: (-3/2)x^2 i.e. -1.5x^2
And 6: (6/1)x^1 i.e. 6x
We can now add these values up to reach our answer but remember integration is only unique up to a constant. Therefore we add a C to represent a constant. Our final answer is therefore: x^4 - 1.5x^2 + 6x + C