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