Simple,
distribute the [tex] ^{2} [/tex]
It may seem tricky..there is a hidden term in there that most people would forget....
but, it's saying multiply (a-3b)(a-3b) (FOIL)
a*a=[tex] a^{2} [/tex] (First)
-3b*a= -3ab (Outer)
-3b*a= -3ab (Inner)
-3b*-3b= 9[tex] b^{2} [/tex] (Last)
Making it....
[tex] a^{2}+-3ab+-3ab+9 b^{2} [/tex]
Combine terms...
Making your answer, [tex] a^{2} -6ab+9 b^{2} [/tex]
