Which step contains an error using the Commutative Property of Addition in verifying 8+x2+17x−x2+4=10+4x2+10x−4x2+2+7x?

To determine which step contains an error in using the Commutative Property of Addition, we should review each step carefully.

**Step 1:**
\[ 8 + 4 + x^2 - x^2 + 17x = 10 + 2 + 10x - 4x^2 + 4x^2 \]

This step rearranges the terms on both sides. However, both sides should add up correctly:
- Left: \(8 + 4 + 17x + x^2 - x^2 = 12 + 17x\)
- Right: \(10 + 2 + 10x - 4x^2 + 4x^2 = 12 + 10x\)

**Step 1 does not equal.**

**Step 2:**
Rearranged the equation from Step 1:
\[ 8 + 4 + 17x + x^2 - x^2 = 10 + 2 + 4x^2 - 4x^2 + 10x + 7x \]

When checking this step:
- Left: \(12 + 17x + 0 = 12 + 17x\)
- Right: \(12 + 10x + 7x + 0 = 12 + 17x\)

**Step 2 is right.**

**Step 3:**
This reads:
\[ 12 + 17x + x^2 = 12 - x^2 + 17x \]

On evaluating:
- Left: \(12 + 17x + 0 = 12 + 17x\)
- Right has an \( -x^2\) on the right that was not accounted for in the left.

This equation is incorrect because there is a difference in the signs regarding \(x^2\).

Thus, **the error occurred in Step 3**.

The answer is **Step 3**.