The average or mean can be calculated by the sum of the temperatures divided by the number of measurements. So:
[tex]\begin{gathered} A=\frac{-5.3+3.6+6.7+0+10-4.3+x}{7}=1.2 \\ \frac{10.7+x}{7}=1.2 \\ 10.7+x=7\cdot1.2=8.4 \\ 10.7+x=8.4 \\ x=8.4-10.7=-2.3 \\ x=-2.3 \end{gathered}[/tex]So, the temperature of Sunday was -2.3°F.