Explanation:
For this problem you may apply the law of cosines,which is an extension of Pythagoras's Theorem. If ABC is our triangle, then:
BC^2 = AB^2 + AC^2 -[ 2*AB*AC*cos(A) ]
I used the right parantheses to avoid any confusion.
Thus we have:
BC^2 = 9 + 16 - [2*3*4*(1/2)]
BC^2 = 25 - 12
BC^2 = 13
BC = sqrt(13)
You may also use this theorem to find the angle formed by 2 sides. This image might help: