Question 945474
{{{drawing(300,300,-2,5,-2,5,grid(1),circle(0,0,0.15),circle(3,1,0.15),circle(1,4,0.15),line(0,0,3,1),line(3,1,1,4))}}}

Using B as the origin, then the vector BA becomes (-3,-1) and vector BC becomes
(-2,3).
You can use the dot product to solve for the angle since
{{{BA*BC=abs(BA)abs(BC)cos(theta)}}}
{{{-3(-2)+(-1)(3)=sqrt(9+1)sqrt(4+9)cos(theta)}}}
{{{6-3=sqrt(10)sqrt(13)cos(theta)}}}
{{{cos(theta)=3/sqrt(130)}}}
{{{theta=74.7}}}