-28/(-9-3) * (-3/(-28)+1/(-4))=
-28/-12*(3/28-7/28)=7/3*-4/28=-1/3
(sina+3sina-4sin^3a)/(cosa+4cos^3a-3cosa)=4sina(1-sin^2a)/2cosa(2cos^2-1)=
=2sinacosa/cos2a=sin2a/cos2a=tg2a
(x+1)^2 - (x-3)(x+3) = 7
(x+1)^2 - (x^2 - 9) =7
x^2 + 2x - 1 -x^2 + 9 =7
2x = -3
x = -1.5