a) you're right, the resultant angle = 45 deg (pi/4 rad) inductive.
b) what is the impedance (hypotenuse) with a resistive side of 1000 ohms, and an inductive side of 1000 ohms (2000 ohm Xl - 1000 ohm Xc) @ 45 deg angle? 1000 ohm x 1.414, you're right
c) to get pure resistance, the reactances must cancel each other, Xl = Xc. this is resonant freq. so set Xl = 2 x pi x F x L equal to Xc = 1/ 2 x pi x F x C and solve for F.