Answer:
A = -6; B = 15
Explanation:
4x-Ay=15
-4x+6y=B
This is your system of equations. For it to have infinitely many solutions, the lines have to be the same. This means that if you multiply one equation by a constant, you should be able to get the other one.
With this system, you have 4x in one equation and -4x in the other. You can use this to figure out the constant. Because -4 is, well negative four, you know that the second equation is the opposite of the first; everything is multiplied by -1.
That means A = -6 and 15 = -B (The other coefficients and constants of the equations also have to be opposites.), and you can solve for B now. Divide by -1 to get B=15.
Hope I could help!