Sara's calculations contain an error. Let's go through the steps she provided and identify the mistake.
**Step 1:**
In this step, Sara correctly set up the expression by rewriting the subtraction of the two polynomials:
\[
2x^3 - 7x^2 + 4x - 9x + 5x^3 - 9x^2
\]
**Step 2:**
In this step, she correctly combined like terms, but the order in which she did it is a bit confusing:
\[
2x^3 - 5x^3 - 7x^2 + 9x^2 + 4x - 9x
\]
It looks like she made an error with the signs when she wrote \(-5x^3\). In fact, when subtracting \(-5x^3\), it becomes \(+5x^3\), so it should be:
\[
2x^3 + 5x^3 - 7x^2 + 9x^2 + 4x - 9x
\]
**Step 3:**
From her expression in Step 2, Sara combines the coefficients incorrectly. The correct combinations should be:
- For \(x^3\): \(2x^3 + 5x^3 = 7x^3\)
- For \(x^2\): \(-7x^2 + 9x^2 = 2x^2\)
- For \(x\): \(4x - 9x = -5x\)
Putting all together correctly would yield:
\[
7x^3 + 2x^2 - 5x
\]
### Conclusion
Sara made her error in Step 2 by misapplying the subtraction of \(-5x^3\) and also in combining like terms. The final answer should be:
\[
7x^3 + 2x^2 - 5x
\]
Her friend should tell her that she did not accurately carry out the subtraction and therefore should recalculate the result.