Explanation:
x = by - 3/2
x + 3/2 = by
y = x/b + 3/(2b)
now compare this to the first equation :
y = 2x + 3
the system has infinite solutions, if both equations are actually identical.
and they are only identical, if b = 1/2
y = x/(1/2) + 3/(2 × 1/2) = 2x + 3