Someone please let me know if I did this wrong! This just seems easier:
P = (Va-Vb)^2/Z so P = (-500 + j500)^2/50<90 = (707<135)^2/50<90 = 499990.41<270/50<90
This equals: 10000<180 i.e. magnitude is 10,000MVAR.
I realize I did not include the "KV" in there, but it's there!
This eliminates having to figure out the current in the transmission line, since you know the impedance of the transmission line. I'm not sure if this would work for all scenarios, but anyway you cut it, you will get the same answer.
That is finding power by V*I or V^2/Z or I^2*Z etc.