When integrating, u-substitution is used to make things easier to work out. In u-substitution, the "u" is generally determined if its derivative is in the expression. That way when integrating with respect to "u", you can just replace the derivative of the u with "du" and easily integrate.
For this question, it would be easiest to let:
[tex]u=2x^3[/tex]
Let's take the derivative of u and see if we can work something out:
[tex]du = 6x^2[/tex]
We have an x² in the integral expression, and therefore, it's valid for us to let u = 2x³. Thus, your answer is A.
