1.9-(5 3/4-(x+1))=3.15

1,9-(5_3/4-(х+1))=3,15
19/10-(23/4-(х+1))=315/100
-(23/4-х-1)=315/100-19/10
х+1-23/4=(315-190)/100
х=125/100-1+23/4
х=(125-100+575)/100
х=600/100
х=6

или
1,9-(5_3/4-(х+1))=3,15
1,9-(23/4-(х+1))+3,15
1,9-(5,75-х-1)=3,15
1,9-5,75+х+1=3,15
х=3,15-1,9+5,75-1
х=6