(430+х)-80=720

430+x-80=720

x=720-430+80

x=370                  

(430+370)-80=720

              720=720

400-(х-300)=200

400-x+300=200

x=400+300-200

x=500                   

400-(500-300)=200

                200=200

Verified answer

1)430+х-80=720

   х=720+80-430 

   х=370

2)400-х+300=200

   700-200=х

   х=500