Calculating a length (triangle)?

Since the the two sides are equal and angle BOD is a right angle that means your triangle is an isosceles right triangle. The hypotenuse BD is now equal to the square root of the sum of the legs, i.e., c = V (a^2 + b^2) , but a = b so, c = V (2*(60)^2 , c = 60 * (square root of 2), c=84.85