m(4x+5)+n(3x-4)=17x-2
(4m+3n-17)x=4n-5m-2
当4m+3n-17=4n-5m-2=0时,不论x何时取何值都成立
4m+3n-17=0 （1）
4n-5m-2=0 (2)
(1)*5+(2)*4
20m+15n-85+16n-20m-8=0
31n-93=0
n=3
m=(4n-2)/5=2
mn=6