1 способ.

tgB=AC/BC ⇒ BC=AC/tgB=9/(3/√7)=3√7

AB=√9²+(3√7)²=√81+63=√144=12

2 способ

CosB=1/√1+tg²B=1/√1+(3/√7)²=1/√1+(9/7)=1/√(16/7)=1/(4/√7)=√7/4

SinB=√1-Cos²B=√1-(√7/4)²=√1-(7/16)=√9/16=3/4

SinB=AC/AB ⇒ AB=AC/SinB=9/(3/4)=12

Verified answer

tgB=AC/BC,следовательно, ВС=9/tgB=3* кв корень  из 7.
 АВ^2=AC^2+BC^2=81+63=144.

 АВ=12 .