The 20th percentile is the cutoff value \(k\) such that \(\mathbb P(X\le k)=0.20\).
Because \(X\sim\mathcal N(50,7)\), you can write
\(\mathbb P(X\le k)=\mathbb P\left(\frac{X-50}7\le\frac{k-50}7\right)=\mathbb P\left(Z\le\frac{k-50}7\right)=0.20\)
Consult a table of \(z\) scores to find the corresponding critical value and you'll find this value to be about \(-0.8416\). So, you have
\(\frac{k-50}7=-0.8416\implies k\approx44.11\)
