Hello!
The length of side BC can be found by using the Law of Sines. This law states,
a / sin A = b / sin B = c / sin C.
Given:
Angle A = 51.2 degrees
Angle B = 60.3 degrees
Side AC (or B) = 21 cm
Side BC (or A) = ? cm
Let's plug in these values into the formula to find the missing side length.
a / sin (51.2) = 21 / sin (60.3)
a / sin (51.2) = 24.1759... (multiply both sides by sin(51.2)
a = 24.1759 · sin (51.2)
a = 18.841...
18.841 can be rounded to 19.
Therefore, the length of side BC is equal to 19 centimeters.