Since the airplane is travelling at a constant height, then your j component stays the same.
Now, you know that the airplane is travelling at a constant velocity. You are given that in 30 seconds, it travels $\displaystyle 9.24 \cdot 10^3$ metres. So, $\displaystyle v = \frac{d}{t} = \frac{9.24 \cdot 10^3 \ \text{metres}}{30.0 \ \text{s}} = 308 \ ms^{-1}$
So how far will it have travelled in 70 seconds? This will be your i-component.
Now use that position vector and find its magnitude and angle with respect to the x-axis.