using global variables in equation driven curves

hello,

i defined two variables with equations as follows:

"r"=40

"k"=1.5

then i want to use these variables in equation driven curves but it turns out red (error) and i dont know why. for example:

x=     "r"*t

y=     "r"*"k"*t

anyone please tell me how to use global variables in equation driven curves?

thanks in advance...

SolidworksSketching