Define SI unit of electric charge ?

Explanation

One Coulomb is that charge which repels an equal and similar charge with a force of `9 xx 10^(9)N`, when placed in vacuum at a distance of one meter from it.
`F=1/(4pivarepsilon_(0))(q_(1)q_(2))/r_(2)`
if`" "q_(1)=q`
`r=1,m`
`F=9 xx 10^(9)N`
then,`" "cancel(9 xx 10^(9))=cancel(9 xx 10^(9)) xx (q xx q)/((1)^(2))`
`q^(2)=1`
`q=+-1C`