Which equation can be used to find the length of ? (10)sin(40o) = ac (10)cos(40o) = ac = ac = ac

The
ABC is a right triangle.
according to the figure, the length AC is the opposite side relative to the angle 40°, 
for that, the main formula is 

sin40° = opposite side / hypotenuse

opposite side =AC, and hypotenuse = 10 in
 
therefore,  sin40° = AC  / 10, and then  AC = 10 sin40°
so for finding the value of AC, we can use   AC = 10 sin40°

look at the image for more explanation