xex and ex x are the same thing
I wonder if the question relates to how you set up integration by parts
integration by parts: ∫v du = uv - ∫u dv
v = x, dv = 1
du = ex , u = ex
∫ = ex x - ex = ex (x - 1) + C
Do it the other way:
v = ex, dv = ex
du = x, u = x2/2
∫ = ex *x - ex*x3/6 This is (obviously?) not going to work. (Look up the rule for choosing v and du when integrating by parts)