Let's call the number we're looking for [tex] y [/tex]. We know that [tex] y [/tex] is twice as much as [tex] x [/tex].
Twice as much as a number simply means the double of that number, and in turn the double of a number means to muliply that number by 2.
So, in order to obtain [tex] y [/tex], you have to multply [tex] x [/tex] by 2. In formula, you have
[tex] y = 2\cdot x[/tex] or simply [tex] y = 2x [/tex]
