There are 2 methods you are taught in Middle School.
Elimination and Substitution.
In order to have a function we can't repeat any value in the x, therefore in order to have a function represented in the table the question mark must be 2
Step 
<u>Find the length of the side MN</u>
we know that
Applying the Pythagorean Theorem

Solve for MN

in this problem

Substitute in the formula above



Step 
<u>Find the value of cos (M)</u>
we know that
in the right triangle MNL


Substitute



therefore
The answer is
The value of cos(M) is equal to 