a² -2a -8 = 0
Move the non-x term to the right
a² -2a = 8
Divide equation by coefficient of a^2 (not necessary).
Take the coefficient of x (which is -2)
divide it by 2
square it
add it to both sides of the equation
-2 / 2 = -1
-1 ^2 = 1
a² -2a +1= 8 +1
(a -1) * (a-1) = 9
Take the square root of both sides
(a-1) = 3
a = 4
Remember that square root of 9 ALSO = -3
(a-1) = -3
a = -2