Ok, I'll explain it to you, but I copied it wrong. This is A2-C2 = 2B.

Solution: sinacosc = 3coasinc

∴sinAcosC+cosAsinC=4cosAsinC

that is

sin(A+C)=4cosAsinC

That is, sinB=4cosAsinC.

∴sinB/sinC=4cosA

But from the sine theorem

sinB/sinC=b/c

∴b/c=4cosA

①

Derived from cosine theorem

a2=b2+c2-2bccosA

Also known.

a2-c2=2b

And b≠0

∴b=2+2ccosA

②

Solve with ① ② formula.

b=4

Hope to adopt