To find f(5) you just replace "x" with "5" in the formula:
That should be easy. You shouldn't even need a calculator.
The second problem asks you to solve an equation: so add 1 to both sides: , and then divide both sides by 4: . Now, for what angles is cosine equal to 1/2?
For start by subtracting 50 from both sides of the equation.
I'm not clear on your last question:
"f(40) -- I'm supposed to identify where the sinusoidal graph has a value of 40 at 4 different points."
Are you to evaluate f(40), which has a single value, or are you to find 4 different solutions of f(t)= 40?