the answer:
For more explanation please look at the image.
according to the image, the length of line segment AC can be found as
the following method shows:
sin(40°)= opposite side / hypotenuse, opposite side = b, hypotenuse = AB= 20cm
sin(40°)= b / 20 and this applies, b = 20 x sin(40°) = 0.64*20 = 12.85 cm
AC= 12.85 cm