built in function fuzairth question

x = -10:0.1:10;
f1 = trapmf(x,[-2 0 0 2]);
f2 = trapmf(x,[-1 2 2 5]);

plot(x,fuzarith(x,f1,f2,'prod'));
title('A * B');

I'm trying to plot this graph without using the built-in function, here's my thought process:

first off, fuzzy multiply works by taking the lower and upper x-bounds of f1, lets say [a b] and of f2, [d e] of the same iteration as my set x-iteration (201 in my case) and then finds

min([a*d, a*e, b*d, b*e]) = lower;

max([a*d, a*e, b*d, b*e]) = upper;

then plot that for the 201 y-values from 0 to 1 in same increments.

I have no idea how to do this I've tried so many different ways but I keep running into either dimension errors or just straight up wrong answers!!

please help!! ANY help will be appreciated greatly.. thank you