the temperature in degree celcius in a factory during the course of a working day from 6am to 10pm, can be modelled by the function

H(t)=18+6sin(pi/8-5pi/4)for6< t < 22
- -
where t is the time in hours on 24 hour clock.
what are the highest and lowest temp. during the working day.

Your trig function part of your equation has no variable, so H(t) is a constant value of 18.38

Do you have a typo?

Assuming there is a variable t in there somewhere, the max value would be 18+6 or 24
and the minimum would be 18-6 = 12

Since I don't know your "angle" I cannot determine when that would happen