Can someone show me how to do this?

For this you've gotta use you trig ratios, often called SOH CAH TOA 

SOH- sine of an angle = opp side/ hypotenuse (longest side) 
CAH- cos of an angle = adjacent side / hyp 
TOA - tan of an angle = opp side / adj side 
- opposite means opposite to the angle given 
-adjacent means right next to angle given 
- hypotenuse is the longest side (the diagonal one) 

so for this question, we're given the angle 30 and you want to find ab which is the hypotenuse (longest side) 
 you're also given the side opposite to the angle. based on what we're given, we must use SOH 

so sin30= 2/hyp 
sin30(hyp)=2 
hyp= 2/sin 30