Find the values of a and b for whic the following system of linear equations has infinite number of solutions:
2x - 3y = 7
(a + b) x - (a + b - 3) y = 4a + b
a = -5, b = -1
Find the values of A and B for which the following pair of linear equations infinitely many solutions:
2x+3y=7
(a+b)x+(2a-b)y=21