Answer:
AB would be 21
Explanation:
Sides BF and DF equal 8, because triangles BFC and DFC are both right triangles and they have two equal sides (hypotenuses are equal and both triangles share side FC), triangles BFC and DFC are congruent triangles, which means side DC is equal to BC. Since right triangles ADC and ABC share side AC and BC = DC, they are congruent triangles, meaning AB will equal AD, which is 21.