(12) -9+2a-8a= (13) -3a-5-2a= (14) (-9a)-(-8a)+3a= (15) -7x+4+2x-8= (16) -3x-2+5-4x= (17) -4-5x-1-3x= (18) (x)/(2)-3-(2)/(5)x+8= (19) -(x)/(3)-2+(x)/(4)-4= (20) -x+(1)/(4)+(3)/(7)x-(5)/(6)=

(12) $-9+2a-8a=-6a-9$<br />(13) $-3a-5-2a=-5a-5$<br />(14) $(-9a)-(-8a)+3a=-9a+8a+3a=-2a$<br />(15) $-7x+4+2x-8=-5x-4$<br />(16) $-3x-2+5-4x=-7x+3$<br />(17) $-4-5x-1-3x=-8-8x$<br />(18) $\frac {x}{2}-3-\frac {2}{5}x+8=\frac {5x}{10}-\frac {4x}{10}+5=\frac {x}{10}+5$<br />(19) $-\frac {x}{3}-2+\frac {x}{4}-4=-\frac {4x}{12}-\frac {24}{12}+\frac {3x}{12}-\frac {48}{12}=-\frac {x}{12}-\frac {72}{12}=-\frac {x}{12}-6$<br />(20) $-x+\frac {1}{4}+\frac {3}{7}x-\frac {5}{6}=-\frac {42x}{42}+\frac {3x}{42}-\frac {35}{42}=-\frac {39x}{42}-\frac {35}{42}=-\frac {39x+35}{42}$