Give regular expression on language

Question

What is a regular expression for $L_1=\{a^{2n} b^{3m+1} \mid n \geq 1, m \geq 0\}$?

Answer 1

Hint: Write $L_1$ as a concatenation of two languages: $\{ a^{2n} \mid n \geq 1\}$ and $\{b^{3m+1} \mid m \geq 0\}$.