Twice a number decreased by eight is zero

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The phrase a number means an arbitrary variable. We can pick any letter a-z except for i and e.

Let's choose x.

Twice a number:
2x

decreased by eight
2x - 8

is means equal to. Set 2x - 8 equal to zero for our algebraic expression:
2x - 8 = 0

If the problem asks you to solve for x, add 8 to each side:
2x = 8

Divide each side by 2:
x= 4