考慮實數二階方陣$\left[{\begin{array}{l l}{a}&{b}\\ {c}&{d}\end{array}}\right]$，若${\left[\begin{array}{l l l}{0}&{1}\\ {1}&{0}\end{array}\right]}{\left[\begin{array}{l l l}{a}&{b}\\ {c}&{d}\end{array}\right]}{\left[\begin{array}{l l}{1}&{0}\\ {0}&{-2}\end{array}\right]}={\left[\begin{array}{l l l}{3}&{-4}\\ {-9}&{-7}\end{array}\right]}$，則 $c-2b$ 的值為何？