Hello Alan, In order to evaluate that function you have two options (I suppose you are in RPN):
1. You can use DEF.
'FUNC(X)=SQ(X)/(3*SIN(X))' [ENTER] [leftShift] DEF
DEF is associated to key [2]
to run press [3] [VAR] FUNC and you get 21.258502...
2. If you want, you can evaluate the function as a program, you don't need variables for a small function, just manipulate the stack.
« DUP SQ SWAP SIN / 3 / » 'FUNC' [STO]
again, to run press [3] [VAR] FUNC and you get 21.258502...