Hey guys just asking a question about transformations of trigonometric graphs

h = Hcos (tπ/6)

Find t when h = 2.5

H = 3 (height of the high tide in meters)

T = 12

a high tide of 3m occurs at 10am

so I went

2.5 = 3cos (tπ/6)

2.5/3 = cos (tπ/6)

0.833= cos (tπ/6)

Then you take cos^-1 (0.833)

= 33.65/180 x 23 = 4.3 (to convert to hours)

4.3 x 6 = tπ

25.8/π = t

t = 8.21

Yeah I think I'm doing it wrong =(

Thanks for any help.