[Cyberconvict]

The question wants me to calculate the "second electron affinity" of oxygen from the following data:
"First electron affinity" of oxygen: -141 kJ/mol
Lattice energy of K₂O: 2238 kJ/mol

The following it says to find in tables and appendix, so here's the rest of what I believe I need:
?Hf[K₂O(s)]: -363.2 kJ/mol
?Hf[K(g)]: 89.99 kJ/mol
?Hf[O(g)]: 247.5 kJ/mol
I₁1(K): 419 kJ/mol

This is the formula I've been trying to use,
?Hf[K₂O(s)]=?Hf[K(g)] + ?Hf[O(g)] + 2I₁1(K) + E₁(O) + E₂(O) - ?H_Lattice

Thus plugging in the numbers,
-363.2 = 89.99 + 247.5 + 2(419) + (-141) + E₂ - 2238

This comes out to E₂ = +840 kJ/mol, but the back of the book says +750kJ. Where am I going wrong?

[Yggdrasil]

You just had one small mistake.  The correct equation should be:

?Hf[K₂O(s)]=2?Hf[K(g)] + ?Hf[O(g)] + 2I₁1(K) + E₁(O) + E₂(O) - ?H_Lattice

Good job on setting the rest up though.