（0.75-3/16)*(2/9+1/3)
=(3/4-3/16)*(2/9+1/3)
=3*(4/16-1/16)*(2/9+3/9)
=3 *3/16*5/9
=5/16
x+3/7=3/4
则x=3/4-3/7=3*(1/4-1/7)=3*(7/28-4/28)=3*3/28=9/28
所以x=9/28