Question

how to deduce the charge on the dichromate ion in Ag2Cr2O7 if silvers charge is +1

Answer 1

2(+1)+2x+7(-2)=0
2+2x-14=0
2x=12
x=6

Answer 2

Charge of Ag2  and Cr2O7 should be same.

Charge of 2Ag is +2 (+1*2=2)

It means +2+X=0. X is an charge of dichromate. So charge of dichromate is -2.