9sin^2a+12sina*cosa+(4sin^a-12sina*cosa+9cos^2a)=0
9sin^2a+12sina*cosa+4sin^2a-12sina*cosa+9cos^2a=0
13sin^2a+9cos^2a=0
13-13cos^2a+9cos^2a=0
-4cos^2a=-13
cosa=+-sqrt{13/4}
9sin^2a+12sina*cosa+(4sin^a-12sina*cosa+9cos^2a)=0
9sin^2a+12sina*cosa+4sin^2a-12sina*cosa+9cos^2a=0
13sin^2a+9cos^2a=0
13-13cos^2a+9cos^2a=0
-4cos^2a=-13
cosa=+-sqrt{13/4}