The two impedances are in series so you draw 10 ohms along the positive x-axis which is the real axis and you draw 15 ohms alond the vertical axis which is the imaginary or jω-axis. The impedance is the vector sum of 10 + j15
XL is ploted along the positive jω-axis and Xc is plotted along the negative jω-axis so:
Rich (BB code):
170 - 90 = 80
so the capacitor in series with the inductor looks like an inductor with a smaller reactance namely 80 ohms. From there it is just like the previous problem