let's say that number is hmmm "a"
now.. twice "a" is 2*a or 2a
4 less than that is " 2a - 4"
now, the product of those two fellows is 286, well ok
a( 2a - 4) = 286
thus
[tex]\bf a(2a-4)=286\implies 2a^2-4a^2=286\implies a^2-2a^2=143
\\\\\\
a^2-2a^2-143=0\implies (a-13)(a+11)=0
\\\\\\
\begin{cases}
a-13=0\implies &a=13\\
a+11=0\implies &\boxed{a=-11}
\end{cases}[/tex]