3a+5b/5a+7b,если 4a+3b=0

3a+5b=3a+1a-1a+5b-2b+2b=(4a+3b)-1a+2b=0+2b-a
5a+7b=5a-1a+1a +7b -4b+4b=(4a+3b)+1a+4b=0+4b+a
3a+5b/5a+7b=2b-a/4b+a