3/7x+33=5/8x
5/8x-3/7x=33
35x/56-24x/56=33
11x/56=33
x/56=3
x=168
(8400+x)/12=(3900+x)/7
7(8400+x)=12(3900+x)
7*8400+7x=12*3900+12x
12x-7x=7*8400-12*3900
5x=12000
x=2400