R(T)=R(T(x,y))
Let's say you're given the point (a,b). Plug that point into T and it returns a number (call it c): T(a,b)=c. Then you plug this number into R to get R(T): R(T(a,b))=R(c)=d.
If you're only given x=a but y remains a variable, then T(a,y) is a function dependent only on the...