Simple enough. Define the two functions. Then subtract them, and plot using fimplicit.
f = @(z) 0.07*z.^2./(0.09 + z.^2);
g = @(y,z) 0.003 + 0.01*42./((y - z).^2 + 42);
fminusg = @(y,z) f(z) - g(y,z);
fimplicit(fminusg)
xlabel y
ylabel z
grid on
It can be a bit simpler if you want to use syms, because then you don't need to define f and g as explicit functions of the to variables.
syms y z
f = 0.07*z.^2./(0.09 + z.^2);
g = 0.003 + 0.01*42./((y - z).^2 + 42);
fminusg = f - g;
fimplicit(fminusg,[-20,20 -.15 .15])
xlabel y
ylabel z
grid on
You can choose a different domain for y and z (in the call to fimplicit) if you wish to expand it. You can see I did that in the second figure.
