Answer provided by our tutors
Our keyboard doesn't have function 'sec'.
sec = 1/cos
You need to type in:
y = 21/(cos(π/4(x+3)))
General functional notation (e.g. h(x) ) is not supported. If you wish to draw a function such as h(x) = 21/(cos(π/4(x+3))), replace h(x) with y (or any other unused variable).
The way you will get the bellow graph.