A=5a+9=2a+12
5a-2a=12-9
3a=3
a=1
B=15y+12=2(5y+21)
15y+12=10y+42
15y-10y=42-12
5y=30
y=30/5
y=6
C=-3(2x+5)-4(-5x-4)=2(6x+7)
-6x-15+20x+16=12x+14
-6x+20x-12x=14+15-16
2x=13
x=13/2
D=(2x+1)(6x+4)=(4x-1)(3x+7)
12x²+8x+6x+4=12x²+28x-3x-7
12x²-12x²+8x+6x-28x+3x=-4-7
-11x=-11
x=1