First, multiply the two terms on the left. To multiply these two terms you multiply each individual term in the left parenthesis by each individual term in the right parenthesis.
#(color(red)(2x) + color(red)(1))(color(blue)(3x) + color(blue)(1))(x + 4)# becomes:
#((color(red)(2x) xx color(blue)(3x)) + (color(red)(2x) xx color(blue)(1)) + (color(red)(1) xx color(blue)(3x)) + (color(red)(1) xx color(blue)(1)))(x + 4)#
#(6x^2 + 2x + 3x + 1)(x + 4)#
We can now combine like terms:
#(6x^2 + (2 + 3)x + 1)(x + 4)#
#(6x^2 + 5x + 1)(x + 4)#
We now use the same process for the two remaining terms:
#(color(red)(6x^2) + color(red)(5x) + color(red)(1))(color(blue)(x) + color(blue)(4))# becomes:
#(color(red)(6x^2) xx color(blue)(x)) + (color(red)(6x^2) xx color(blue)(4)) + (color(red)(5x) xx color(blue)(x)) + (color(red)(5x) xx color(blue)(4)) + (color(red)(1) xx color(blue)(x)) + (color(red)(1) xx color(blue)(4))#
#6x^3 + 24x^2 + 5x^2 + 20x + 1x + 4#
We can now combine like terms:
#6x^3 + (24 + 5)x^2 + (20 + 1)x + 4#
#6x^3 + 29x^2 + 21x + 4#