This is a 30-60-90 triangle which follows a specific rule.
Under this rule; the side containing the 60 and 90 angles is x, the side containing the 30 and 90 angles is x sqr root 3, and the side containing the 60 and 30 angles is 2x.
We are given the side that contains the 30 and 90 angles and therefore we know that is the same as x sqr root 3. Since it’s given as 4 sqr root 3 we can solve for x and get 4 for x.
We need to know the side that contains the 30 and 60 angles. Since this is 2x we can multiply what we got for x by 2. 2*4 is 8.
