If it's not what You are looking for type in the equation solver your own equation and let us solve it.
D( a )
a-20 = 0
a+10 = 0
a-20 = 0
a-20 = 0
a-20 = 0 // + 20
a = 20
a+10 = 0
a+10 = 0
a+10 = 0 // - 10
a = -10
a in (-oo:-10) U (-10:20) U (20:+oo)
-((4*a)/(a+10)) = 12/(a-20) // - 12/(a-20)
-((4*a)/(a+10))-(12/(a-20)) = 0
-4*a*(a+10)^-1-12*(a-20)^-1 = 0
(-4*a)/(a+10)-12/(a-20) = 0
(-4*a*(a-20))/((a+10)*(a-20))+(-12*(a+10))/((a+10)*(a-20)) = 0
-4*a*(a-20)-12*(a+10) = 0
68*a-4*a^2-120 = 0
68*a-4*a^2-120 = 0
4*(17*a-a^2-30) = 0
17*a-a^2-30 = 0
DELTA = 17^2-(-30*(-1)*4)
DELTA = 169
DELTA > 0
a = (169^(1/2)-17)/(-1*2) or a = (-169^(1/2)-17)/(-1*2)
a = 2 or a = 15
4*(a-2)*(a-15) = 0
(4*(a-2)*(a-15))/((a+10)*(a-20)) = 0
(4*(a-2)*(a-15))/((a+10)*(a-20)) = 0 // * (a+10)*(a-20)
4*(a-2)*(a-15) = 0
( a-15 )
a-15 = 0 // + 15
a = 15
( a-2 )
a-2 = 0 // + 2
a = 2
a in { 15, 2 }
| 41z+36=36-41z | | 4(2x+3)=2(4x+6) | | 5x^2-5=-4x-5 | | 0.72*1.5+0.28*20x=(1-x)*0.28*20 | | 1/2m=1 | | y=0.5(450)+300 | | 180=18x-14 | | y=0.5(200)+300 | | y=0.5(150)+300 | | 0.72*1.5+0.28*20x=0.5+(1-x)*0.28*20 | | y=0.5(100)+300 | | 7/4k^-1 | | y=0.5(50)+300 | | -0.2=y/-0.5 | | 3(3a+5)=96 | | (3a/7)+(2/3)=(7/9) | | y=0.5(0)+300 | | 2250=500x+780 | | f=k4x/121x^2 | | 166=6x+16 | | x^2/(x-3)-(5x-6)/(x-3) | | 198=5x+33 | | 2050=280x+710 | | 8n+16.6=1.6 | | X^2-4x-39=6 | | x+3/4=7/9 | | 0.2*(4x+2)=2 | | 188=7x+14 | | (5x+9)(2x-1)= | | 1/5h=3 | | -4x=65 | | 0.2(2x-1)=0.2+0.08 |