Question

SOS I'm terrible at math.

Answer 1

y = -5 cos x is a basic (simple) variation of the basic trig function y = cos x.

The given y = -5 cos x could be re-written as y = -5 cos 1x.
Compare this to the most general form:          y =  a cos bx.

Here we see that the coefficient b equals 1.

There is a rule for finding the period of such a function:  Period = 2pi/b.

Since b = 1 here, the Period here is 2pi/1, or simply 2pi.

Please stop saying "I'm terrible with math."  That does not help you at all.  Say, instead, "With just a little help I could understand this just fine."