Answer:
[tex]\large\boxed{3a^n(a^n+a^{n-1})=3a^{2n}+3a^{2n-1}}[/tex]
Step-by-step explanation:
[tex]3a^n(a^n+a^{n-1})\qquad\text{use the distributive property}\ a(b+c)=ab+ac\\\\=(3a^n)(a^n)+(3a^n)(a^{n-1})\qquad\text{use}\ x^n\cdot x^m=x^{n+m}\\\\=3a^{n+n}+3a^{n+n-1}\\\\=3a^{2n}+3a^{2n-1}[/tex]
