First, you are writing 1/8 without the x in your answer, but your work obviously shows you know there is an x there. So, I am ignoring that.

When we write mixed numbers like 1 2/15, we understand that there is a plus sign between them. Thus, 1+2/15 = 17/15.

1 2/15 x^2 would confuse the math, since we know the 2/15 is being multiplied by x^2 and BEDMAS/PEDMAS rules all.

This is one of the reasons we rarely use mixed numbers in math, sticking to improper fractions.

With respect to the + -1x/8, I would expect my students to simplify this to - x/8.