A student subtracted two polynomials. They asked a friend to check their solution and work. What should their friend tell them? Are they correct? If they are not correct, in which step did the student make an error?

To check the student's work, let's carefully go through the steps of polynomials subtraction and identify any errors.

The original problem is:

\[
(2x^3 - 7x^2 + 4x) - (9x - 5x^3 + 9x^2)
\]

### Step 1: Distributing the negative sign
When we subtract the second polynomial, we need to distribute the negative sign across all terms in the second polynomial:

\[
2x^3 - 7x^2 + 4x - 9x + 5x^3 - 9x^2
\]

The student wrote:
\[
2x^3 - 7x^2 + 4x - 9x - 5x^3 + 9x^2
\]

**Error Found**: In the student's version of Step 1, the term \( -5x^3 \) is written with a negative sign instead of a positive. The correct term should be \( +5x^3 \).

### Corrected Step 1:
\[
2x^3 - 7x^2 + 4x - 9x + 5x^3 - 9x^2
\]

### Step 2: Combining like terms
Now, we combine like terms correctly:

- For \(x^3\): \(2x^3 + 5x^3 = 7x^3\)
- For \(x^2\): \(-7x^2 - 9x^2 = -16x^2\)
- For \(x\): \(4x - 9x = -5x\)

So we obtain:
\[
7x^3 - 16x^2 - 5x
\]

The student's Step 2 had incorrect calculations due to the error in Step 1.

### Final Answer:
The student's final answer was:
\[
-3x^3 + 2x^2 - 5x
\]

This is incorrect due to the error in Step 1 that led to incorrect combining in Step 2.

### Conclusion:
The friend should tell the student that they made an error in Step 1 regarding the sign of the \(5x^3\) term. The correct final answer after properly combining the like terms should be \(7x^3 - 16x^2 - 5x\).