Find the solutions of x: 2x(x + 4)(x - 20) = 0

x = 0, -4 or +20

Since the left hand side of the equation is already factored for us, we know the following:
Either
2x=0
or
x+4=0
or
x-20=0
If any one of these is zero, then the entire left hand side is zero.
Now we solve for x in each of the above cases:
2x = 0 means x = 0
x+4=0 means x = - 4
x-20=0 means x = 20

You can check your answer by plugging in any of the x values we obtained into the original equation and you will end up with 0.
For example:
Let x = -4
2(-4)(-4+4)(-4-20) = 0
-8(0)(-24) = 0
0 = 0

2x(x + 4)(x - 20) = 0?
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Let's say if x.y = 0, then either x = 0 or y = 0 since 0.y = 0 and x.0 = 0
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If 2x.(x + 4)(x - 20) = 0, then either 2x = 0, x + 4 = 0, or x - 20 = 0

Hence, either x = 0, x = - 4, or x = 20
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Hope this helps!

2x=0
x=0
and
x+4=0
x= - 4
and
x-20=0
x=20

The left side is 0 when any of its factors is 0 (zero factor law) so x = -4, 0, 20