For 19 a):
we know
px + qy + r = 0 (eq1)
x^2 +y^2 = d^2
Notice that y is not i the equation of 19a). So to remove y make a substitution.
Make y the subject in eq1
=> y = (-r - px)/q
Now substitute.
A lot parametric equation have a similar format. Often just involving substitution.