Answer
$$
976 \mathrm{\ nm} .
$$
Work Step by Step
At "decoupling" (when the universe turns out approximately "transparent"),
$$
\lambda_{\max }=\frac{2898 \mu \mathrm{m} \cdot \mathrm{K}}{T}=\frac{2898 \mu \mathrm{m} \cdot \mathrm{K}}{2970 \mathrm{K}}=0.976 \mu \mathrm{m}=976 \mathrm{nm} .
$$
