I'm assuming this is homework. Let me give you a couple of hints.
1. You can make a function like this
q = @(b)integral(PM,0,b);
Then, as you know,
But you can make a useful function like this
This function g is zero when integral(PM,0,b) is 1/5 of integral(PM,0,5). Then you might want to look at
That function is zero when integral(PM,0,b) = (2/5)*A. And so forth...
2. FZERO is a function that might interest you.