Past exam of the mathematics course of the University of Cambridge / 2024 / iii / Paper 128 / 1 / a / Solution

First form the singleton set $\{b\}=F_1(b,b)$ and then use $F_2$ to take a set union:The unused second argument of $F_2$ may be any term. Since $x\cap c=x\setminus (x\setminus c)$, a term using only the prescribed operation symbols isAfter substituting the displayed term for both occurrences of $U$, this is literally a term in $F_1,F_2,F_3$, and its value is $(a\cup \{b\})\cap c$.