(12-0.75x):[7.8-0.75x(x-2)]=5:3
5[7.8-0.75x(x-2)]=3(12-0.75x)
39-3.75x(x-2)=36-2.25x
3.75x*x-9.75x-3=0
5x*x-13x-4=0
x=