Hello,

Perfect thing for the first question !

For the decreasing : a function decreases when its derivative is negative.

Thus, you have to solve for .

A quick way to do is to say that it's negative if and only if the signs for 3x and 4-x are different, that is to say that if 3x is positive, then 4-x has to be negative, and if 3x is negative, 4-x has to be positive.

Hence the solutions are :

This should give you the set of values for x that you want