(b)

From the above figure,

Initial velocity, $\vec{v}_{ i }=\left(-20 \sin 30^o \hat i-20 \cos 30^{\circ} \hat j \right) m / s$

Final velocity, $\overrightarrow{ v }_{ f }=\left(-20 \sin 30^{o}\hat i+20 \cos 30^o \hat  j \right) m / s$

Change in velocity, $\Delta \vec{v}=\vec{v}_f-\vec{v}_i$

$\Delta \overrightarrow{ v }=\left(-20 \sin 30^{o}{ \hat{ i }}+20 \cos 30^0 j \right)-\left(-20 \sin 30^o \hat{ i }-20 \cos 30^{o}{\hat j }\right)$

$\Delta \overrightarrow{ v }=2 \times 20 \cos 30^{o}{ \hat{ j }}$

$\Delta \overrightarrow{ v }=2 \times 20 \times \frac{\sqrt{3}}{2} \hat{ j }$

$\Delta \overrightarrow{ v }=20 \sqrt{3} \hat{ j }$