create the function x t = 3e −1.5t sin 2t cos 4t in matlab

You can create a function with the specified expression using the following code:

main.m
function [y] = xt(t)
    y = 3*exp(-1.5*t)*sin(2*t)*cos(4*t);
end
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Save the file with any name (let's say myFunc.m) and call it from wherever you want by using myFunc(t).