You first wanna find <BAD, because if AB is perpendicular to AC, then it has to form a 90 degree angle. So 90-56=34 degrees. So now you have a 34 & 63 degrees in the ABD triangle. In a triangle, all angles add up to equal 180 degrees. So 34+63+x=180...and x=83. So <ADB= 83 degrees. Now you want to find angle ADC...which you can just subtract 83 from 180 (because <ADB & <ADC forms 180 degree angle). You will then get 97 as angle ADC. So, the same thing as before, add up 56+97+x=180, because all angles (in the triangle ADC) add up to be 180 degrees. X will then equal 27 degrees. Angle ACB= 27 degrees.