-3x+7y+13x-7y-3 = -3*(-3)+7*(-15,01)+13*(-3)-7*(-15,01)-3*(-3) = 9-105,07-39+105,07+9 = 9-39+9 = -21
7с^4 * 2c + 7c^4 * 1= 7c^5+7c
4•tg^2(3x) - cos^(-2)(3x) = 2
4•tg^2(3x) - (1 + tg^2(3x)) = 2
3•tg^2(3x) = 3
tg(3x) = (+/-) 1
3x = (+/-) π/4 + πn, n є Z
3x = π/4 + πn/2, n є Z
x = π/12 + πn/6, n є Z