First of all note the dx means that we are integrating with respect to x.
So lets integrate each term seperately..
3x^2 ---> (raise the power by one then divide by the new power, so 3) ---> x^3
4x^5 ---> (raise the power by one and then divide by the new power, so 6) ---> 4/6 x^6 --->(simplify fraction) 2/3 x^6
-7 ---> (raise the power by one from x^0 to x^1 then divide by the new power, so 1) ---> -7x
when integrating without boundaries we must add a constant ---> +C
putting it all together we get= x^3+ 2/3 x^6 -7x +C