Question 1197251
 Using the  right triangle formed: Slope AC = -1, Slope BR = 1
BR = √2,  RC = AR =√18 ,  BC = AB = √20 ,  Triangles congruent  
 ∠A  = ∠C = {{{ tan^(-1)(sqrt(2)/sqrt(18))}}} ~  18.435°   
∠B = 180 - 2*18.435  
{{{drawing(300,300,   -6, 6, -6, 6, grid(1), 
circle(-3, 3,0.3),
circle(-1, -1, 0.3),
circle(3,-3, 0.3),
line(-3,3,3,-3),
line(-3,3,-1,-1),
line(-1,-1,3,-3),
line(-1,-1,0,0),
locate(-1.8,-1.2,"B"),
locate(-3.8,3.3,"A"),
locate(2.9,-3.3,"C"),
locate(0.3,0.6,"R"),
graph( 300, 300, -6, 6, -6, 6,0)) }}}