Answer:
sine A = four-fifths
Explanation:
You have a rectangle triangle ABC with hypotenuse AB 35, side CB 28 and side CA 21. The angle C is 90°.
In order to calculate sin A, you take into account that:

The opposite side to angle A is side CB = 28
Hypotenuse = 35
You replace the numeric values of the hypotenuse and side BC in the formula for sinA:

In order to simply the fraction, you divide both numerator and denominator by 7:
hence, the answer is
sine A = four-fifths