We'll have to use the law of cosines to determine the length of the side "a".
a^2 = b^2 + c^2 - 2b*c*cos (b,c) (1)
We know that the side "a" is facing to the angle A, then the side "b" is facing to B and the side "c" is facing to the angle C.
The angle enclosed by the sides "b" and "c" is A.
We know, from enunciation, that A=60 degrees.
Then cos 60 = 1/2
We'll substitute the values of b and c and the cosine of A, into the relation (1).
a^2 = 3 + 4 - 2*sqrt3*2/2
a^2 = 7 - 2sqrt3
a = sqrt(7 - 2sqrt3)
The length of the side "a" is: a = a = sqrt(7 - 2sqrt3).