7 cos a - 6 sin a/3sin a - 5cos a; если tg a = 1

tga=1\\\\ \frac{7cosa-6sina}{3sina-5cosa} = \frac{cosa\cdot (7-6tga)}{cosa\cdot (3tga-5)} = \frac{7-6}{3-5} = \frac{1}{-2} =-0,5