Pleeeeeease help with math!

if you dont help im literally going to faill!! i have to present how to do this question to my class tomorrow and have no idea how to do it!!

natasha is a marathon runner and she likes to train on a 2pi km stretch of rolling hills. The height, in kilometers, of the hills above sea level, relative to her home can be modeled by the funtion
h(d) = 4cos^2 d - 1 where d is the distance traveled in kilometers. At what intervals in the stretch of rolling hills is the height above sea level, relative to natashas home, less than zero?

if you show tell me how to do this i will give you 10 points, and i swear ill answer any of your questions!!

I think the way to answer this question is

when h(d) will be less than zero only than the stretch of the hills will be above sea level but below the level of house.

so h(d) < 0

4 cos sq (d) -1 < 0

4 cos sq (d) < 1

cos sq (d) < 1/4

cos (d) < + 1/2, - 1/2

-1/2 < cos (d) < 1/2

while applying cos inverse, greater than smaller than symbols will be inverted.

120 degrees > d > 60 degrees

Its your answer.

h(d) = 4cos^2 d - 1
For h(d)<0
then 4cos^2 d - 1<0
cos^2 d < 1/4
+/-cos d < 1/2
cos d < +/-1/2
+2/3*Pi > d > +Pi/3, and
+4/3*Pi < d < 5/3*Pi