(a) The intersection point implies that the pair of x and y values (the coordinates (0,3)) satisfy both equations. So, if we plug x=0 into each equation, the y values will be the same for each equation. The intersection point is the y-intercept for both equations.
(b) Let's pick some values for a and b first to satisfy (0,3). b0=1 for all b, so a=3. Let b=2 (arbitrary value), then y=3(2)x. This is an increasing function. The second equation has to be decreasing but y=3 when x=0, so c=3. I think y=c(d)-x for the second equation, because, like the first equation, y=c(d)x would also be increasing. So let d=5, y=3(5)-x. This equation decreases exponentially.
y=3(2)x and y=3(5)-x are examples.