I dont know how to get the correct answer

Answer:
[tex]42a^{2}[/tex]
Step-by-step explanation:
Split them into sections!
First, [tex]3a[/tex] x [tex]5a[/tex] = [tex]15a^{2}[/tex]
Next, [tex](3a+3a+3a)(3a)[/tex] = [tex]9a[/tex] x [tex]3a[/tex] = [tex]27a^{2}[/tex]
Add [tex]15a^{2}[/tex] and [tex]27a^{2}[/tex] to get [tex]42a^{2}[/tex]
Hope I helped!