Question

What is 2a squared if a=3?

Answer 1

2(3)^2= 18 that is with a(3) squared or 2(3)=6 and 6squared or 6^2 = 36.

Answer 2

If You mean is :
$2a^{2}~~~~~~[just\ subtituted\ a\ into\ 3]$

$2a^{2}\\=2(3)^{2}\\=2(3 \times 3)\\=2(9)\\=18$