Question 43472
Be aware of minus signs! You ahve to invert the signs of the whole second poly:

{{{(7x^2+3x-9)-(-x^2+8x-3)}}}
{{{7x^2+3x-9 + x^2 - 8x + 3}}}

Then, you sum every term that has the same literal part:

{{{8x^2 -5x -6}}}