Hi guys, Thanks for all help in advance.

A question requires me to find t in seconds.

h = 13 m

13 = -10 cos (πt/15) + 11.

13-11/-10 = cos (πt/15)

cos^-1(1/5) = 78.5

78.5 = πt/15

...

is what im doing right so far? im required to convert 78.5 degrees to seconds, im unsure how to do this. =(