Our keyboard doesn't have function 'sec'.

However since: 

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.

Click to here see the graph in the solver.