Answer:
x = 2+
Explanation:
1) FADH2 + Q => FAD + QH2
Since H is added to Q
=> Reactant reduced is Q
(2) Balancing charges on both sides of the equation gives:
QH2 + 2 cyt c(Fe3+) => Q + 2 cyt c(Fe2+) + 2 H+
Thus x = 2+
It looks like part of the Periodic Table. Do you have to write down the most common Periodic Table elements? If so, then that's probably what you should do.