Because vertical angles are congruent, angle 78 is congruent to angle x: x=78.
First
Mutliply -2 and -3
Then
Add 4+6+5
Finally
4+6=10
And then
10+5=15
I think it is D or it could be c?
The law of cosines is used for this purpose. It tells you
c² = a² + b² - 2ab×cos(C)
Solving for the angle C gives
C = arccos((a² + b² - c²)/(2ab))
C = arccos(( 17² + 15² - 19²)/(2×17×15))
C = arccos(153/510)
C ≈ 72.5424°
The measure of the angle opposite the longest side is about 73°.
