I'll get you started by noting that ABF is a triangle with interior angle sum of 180 degrees and is an isosceles triangle with two of the sides being 6 cm.
Since the interior angle sum is 720 degrees we know that angle FAB is equal to 720/6 = 120 degrees. We can use the cosine rule to get the length of BF by noting that:
c^2 = a^2 + b^2 - 2abcos(C) (cosine rule). Note that 120 degrees = 2*pi/3 in radians.