(10-6)200+(10-6-x)(200+50x)+(4-6)(600-200-200-50x)=1250
800+(4-x)(200+50x)+(-2)(200-50x)=1250
800+800+200x-200x-50x²-400+100x=1250
1200+100x-50x²=1250
-50x²+100x-50=0
x²-2x+1=0
x=1
(10-6)200+(10-6-x)(200+50x)+(4-6)(600-200-200-50x)=1250
800+(4-x)(200+50x)+(-2)(200-50x)=1250
800+800+200x-200x-50x²-400+100x=1250
1200+100x-50x²=1250
-50x²+100x-50=0
x²-2x+1=0
x=1