The angles of a triangle all add up to 180 degrees. One side has an angle of 90 (right angle). So to find the others you would subtract 180 from 90 and divide it by 2. And you will get 45, so both acute angles will be 45 degrees.
If you subsitute 21 for x you will get 2*21 + 3 = 45, which is true (42 + 3 = 45)
A negative times a negative results to a positive number, but when two negative numbers are added to each other, it still results to a negative number.
