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
