Step by step
The question
Question: expand (x − 4)(2x + 1)(3x − 2).
Expand means: multiply out all the brackets.
1 Multiply the first two
First the first two brackets: (x − 4)(2x + 1).
The third waits.
Careful!
We can't just multiply the first parts and the last parts.
Every part meets every part.
Each part of the first bracket multiplies the whole second bracket.
The parts of the first bracket are x and −4.
Each one takes the whole second bracket.
What is in front of a bracket multiplies every part inside.
So we write each part times what is in front.
Watch the signs.
x times 2x is 2x².
x times 1 is x.
(−4) times 2x is −8x.
(−4) times 1 is −4.
Terms are the parts between the + and − signs.
Like terms are the same kind: same letters, same powers.
All the x terms go together.
Each colour is one kind.
x − 8x is −7x.
It is like 1 apple take away 8 apples: that is 7 apples.
So the expression is 2x² − 7x − 4.
2 Times the third bracket
Now this answer times the third bracket, (3x − 2).
Each part of the long bracket meets every part of the third.
Each part of the first bracket multiplies the whole second bracket.
The parts of the first bracket are 2x² and −7x and −4.
Each one takes the whole second bracket.
What is in front of a bracket multiplies every part inside.
So we write each part times what is in front.
Watch the signs.
2x² times 3x is 6x³.
2x² times (−2) is −4x².
(−7x) times 3x is −21x².
(−7x) times (−2) is 14x.
3 Group like terms
Terms are the parts between the + and − signs.
Like terms are the same kind: same letters, same powers.
All the x² terms go together.
Each colour is one kind.
−4x² − 21x² is −25x².
14x − 12x is 2x.
It is like 14 apples take away 12 apples: that is 2 apples.
So the expression is 6x³ − 25x² + 2x + 8.
4 Check
Let's check.
We pick a number: x = 2.
We put it into the start: −40.
Now into our answer.
Both give −40.
Same value, so 6x³ − 25x² + 2x + 8 is right!
Let's look back.
First: multiply the first two.
Then: times the third bracket.
Then: group like terms.
Last: check.