substitute your point (3,-2) on both equations:
A(3)+b(-2)=18 <--equation 1
a(3)-b(-2)=6 <--equation 2
add both equations, you'll get
6A=24
A=4
substitute the value of A in any equtions
equation 1: (4)(3)-2b=18
simplify and solve for b, you'll get b=-3
thus, A=4 & B=-3
