Rewrite the expression (x+3)(3x+8)-3x(x+3)

Multiply (x+3)(3x+8)      x times 3x =3x^2 then multiply x times 8 = 8x
                                 keep multiplying 3 times 3x =9x  then 3 times 8=24 
                          then multiply the -3x(x+3)=  -3x^2-9x
your hole expression will look like this 3x^2+8x+9x+ 24-3x^2-9x

if you need to simplify you can do by adding or subtracting them.
for example you can add 8x+9x= 17x