Element that is the "best" at removing electrons from other atoms.

Fluorine

Explanation:

Fluorine is the "best" at removing electrons because it has 5 electrons on its second shell. The 5 electrons are really close and are trying to drag in another electron to complete the shell and have all 6 electrons to complete the valence shell.

so here we go

atomic mass

239 + 1 = y + 134 + 3 combine like terms

240 = y + 137             subtract 137 from both terms

240 - 137 = y

103 = y

atomic number

94 = z +54 + 0

94 = z + 54               subtract 54 from both sides.

40 = z

answer