Alright so the sin of anything is opposite/hypotenuse.
we can keep this in mind and use the information we have...
Opposite of <NOM = 3
To find the hypotenuse we will use the pythagorean theorem...
a²+b²=c²
3²+4²=c²
9+16=c²
25=c²
√25=√c²
c=5
Knowing that the hypotenuse is 5, we can use the sin formula and find that...
sin(<NOM)= 3/5
I hope this helps!