This is a special triangle, also known as a 30-60-90 triangle because of each of its angles. For this type of a triangle, the leg that you are given is denoted by "x." So, in our case:
x = 3
The other leg of the triangle is also always equal to x√3. In our case:
b = 3√3
The hypotenuse is always equal to 2x. So, in our case:
a = 6
Therefore, your answer is A, a = 6, b = 3√3.
*Note that these rules only apply to a 30-60-90 triangle - not to all right triangles (nor other triangles).