To start with 21, we will find the last missing angle. We currently have 45 degrees for X, 90 degrees for Y, and Z is unknown.
A triangle adds up to 180 degrees. So to find Z, we will add X, Y, and Z together and set it equal to 180.
45 + 90 + Z = 180
135 + Z = 180
Z = 45
Now we have all 3 angles. Let’s look at the choices:-sinX-cosX-sinZ-cosZ
So there are no choices for Y. We can forget about Y.
Both X and Y are 45 degrees. So all we have to do is simply match sin and cos.
So sinX is equal to sinZ and cosX is equal to cosZ. Because they’ll be sin45=sin45 and cos45=cos45.
Hope this helped!