Question: Error, (in plots:-pointplot) points cannot be converted to floating-point values

Hi, Very new to Maple. This is a math assignment, and well I am not exactly sure what is happening. I have had a buddy to help me, but things are not necessarily working. Using Maple17.
This is what I have so far.

de := sin(x)*(diff(y(x), x))+cos(x)*y(x) = Q;
/ d \
sin(x) |--- y(x)| + cos(x) y(x) = Q
\ dx /
dsolve(de);
Q x + _C1
y(x) = ---------
sin(x)
ab := 75; Q := 2-0.1e-1*ab; M := 4+0.1e-1*ab;
75
1.25
4.75
PS := simplify(dsolve({de, y((1/2)*Pi) = M}, y(x)));
-10 x - 38 + 5 Pi
y(x) = - -----------------
8 sin(x)
z := rhs(PS);
-10 x - 38 + 5 Pi
- -----------------
8 sin(x)
N := evalf(subs(x = .4, z));
8.439521805
a := z, x = 0 .. Pi, y = 0 .. 10; b := plots[pointplot]([(1/2)*Pi, M]); c := plots[pointplot]([.4, N]); d := plots[pointplot]([x0, y0]); plots[display]({a, b, c, d});
z, x = 0 .. Pi, y = 0 .. 10
Error, (in plots:-pointplot) points cannot be converted to floating-point values
Error, (in plots:-pointplot) points cannot be converted to floating-point values
Error, (in plots:-pointplot) points cannot be converted to floating-point values
Error, (in plots:-display) expecting plot structures but received: {c, d, z, plots:-pointplot, x = 0 .. Pi, y = 0 .. 10}