Let E : $\{0,1\}$k $\backslash$times $\{0,1\}$m $7->\{0,1\}$m be a blockcipher known to be a secure PRF$.$

Prove that the function family F$2$ : $\{0,1\}$k $\backslash$times $\{0,1\}2$m $7->\{0,1\}$m specified for all

x$1,$ x$2$ in $\{0,1\}$m and all K in $\{0,1\}$k below is not a secure PRF: