5x²+7x-6=0
(5x-3)(x+2)=0
x=3/5或x=-2
因为-1≤sina≤1
所以：sina=3/5
[sin(3π/2-a)tan(2π-a)]/[cos(π/2-a)cos(π/2+a)]
=cosatana/(-sin²a)
=-1/sina
=-5/3