Rewriting a function in specific terms with Matlab -

i feel there should easy solution problem couldn't find it.

i can explain problem through simple example:
given functions f , a:

f := x(x-3) + 2x(x-3) := (x-3) 

i want write f in terms of should be:

f = (x + 2x)a or f = 3xa 

this easy example, want includes integrations of volumes.

i did find mupad functions "simplify" , "rewrite" not believe can want here. there function in matlab or procedure can automatically?

you wish substitute of x elements in expression? how expect computer know want? computers quite literal things. if tell that

a = x - 3 

then must know that

x = + 3 

therefore,

f = (a+3)a + 2(a+3)a = 3a(a+3) 

only computer can read mind know wish replace of terms.