Describe $R \circ Q$ and $Q \circ R$ in the case where $A=\{1,2\}, R=\{(1,1),(1,2)\}$ and $Q=\{(2,1)\}$

Question

Describe $R \circ Q$ and $Q \circ R$ in the case where $A=\{1,2\}, R=\{(1,1),(1,2)\}$ and $Q=\{(2,1)\}$

I know how to do this type of problems with functions but this is the first time I've seen something like this with sets

Answer 1

It's composition of relations

So $R\circ Q = \{(x,y)\mid \exists a\in A~( (x,a)\in Q\wedge (a,y)\in R)\}$

$$\{(\color{blue}1,1),(\color{blue}1,2)\}\circ\{(2,\color{blue}1)\} = \{(2,1),(2,2)\}$$

Now can you do $Q\circ R= \{(x,y)\mid \exists a\in A~( (x,a)\in R\wedge (a,y)\in Q)\}$ ?

$$\{(2,1)\}\circ\{(1,1),(1,2)\} = \underline{\qquad\qquad}$$