Y=5-4x
3x+5(5-4x)= -9
3x+25-20x = -9
-17x=-34
x=2
A)cos^2a+sin^2a=1
1+tg^2a=1+tg^2a
b)-cos^2a+1=sin^2a
2sin^2a=2sin^2a
c)sin^2a-1=-cos^2a
-cos^2a= -cos^2a
4x^2-4=2(2x^2-2)
5t^2-20st+20s^2=5(t^2-4st+4s^2)
c-27cd^3=c(27d^3)
vu^3+vw^3=v(u^3+w^3)
7х+2+5х-4-12х= 7х+5х-12х=0, 2-4=-2
-7х ..........................