algebra
posted by Algebra PLEASE HELP!! .
The point of intersection of the graphs of the equations of the system
Ax – 4y = 9
4x + By = –1
is (–1, –3). Explain how to find the values of A and B, then find these values.

The point (1,3) must lie on both curves. Thus
A 4*(3) = 9
A +12 = 9
A = 3
and
4*(1) +B*(3) = 1
Solve that for B 
x =  1
y =  3
A x – 4 y = 9
A * (  1 )  4 * (  3 ) = 9
 A + 12 = 9
 A = 9  12
 A = 3 Multiply both sides by  1
A = 3
4 x + B y = –1
4 * (  1 ) + B * (  3 ) =  1
 4  3 B =  1
 3 B =  1 + 4
 3 B = 3 Divide both sides by  3
B = 3 /  3
B =  1