(2c-a)/b=(2sinC-sinA)/sinB

So (COSA-2 COSC)SINB =(2 SINC- Sina) COSB.

B=π-(A+C)

sinB=sin(A+C)，cosB=-cos(A+C)

Then (COSA-2 COSC) SIN (A+C) =-COS (A+C) (2 SINC-Sina)

Simplified to sinC-2sinA=0.

So sinC/sinA=2

I hope it helps you. . .