REALLY EASY, I'M JUST DUMB. BUT PLEASE HELP ME

2x minus y equals 6, x plus y equals negative 3

Which sum or difference identity would you use to verify that cos (180° - q) = -cos q? a.) sin (a -b) = sin a cos b – cos a sin b b.) cos (a -b) = cos a cos b – sin a sin b c.) cos (a -b) = cos a cosb + sin a sin b d.) sin (a + b) = sin a cos b + cos a sin b

Ntriangle abc, m∠a = 35° and m∠b = 40°, and a=9. which equation should you solve to find b?