In triangle abc a is a right angle and m b=45 what is the length of bc

Since A is a right angle and mB = 45°, the right triangle is a 45-45-90 triangle.
This means the following:long leg and short leg have the same value of xhypotenuse is equal to x√2
Since the long leg has been given as 12 ft. The measure of the short leg is also 12 ft. 
hypotenuse = x√2 = 12 √2 = 12 * 1.414 = 16.97 ft  ⇒ this is the measure of BC.