Sanaz tried to subtract two polynomials. Her work is shown below: $\begin{align} &\phantom{=}(-3a^4-7a^2b^2+6b^4)-(2a^4-3a^2b^2+4b^4) \\\\ &=-3a^4-7a^2b^2+6b^4-2a^4+3a^2b^2-4b^4&\text{Step }1 \\\\ &=(-3-7)a^6b^2+(6-2)a^4b^4+(3-4)a^2b^6&\text{Step }2 \\\\ &=-10a^6b^2+4a^4b^4-a^2b^6&\text{Step }3 \end{align}$ **At which step did Sanaz make an error, if at all?** [[☃ radio 2]]