Make this equation equal to zero by subtracting 40 to both sides:
x² + 6x - 40 = 0
You must remember that a polynomial is written like so...
ax² + bx + c
In this case...
a = 1
b = 6
c = -40
To factor you must find two numbers who both add up to b (6) AND multiply to c (-40)
-4 + 10 = 6
-4 * 10 = -40
so...
(x - 4)(x + 10)
To find the zero you must set each factor equal to zero and solve for for x like so...
x - 4 = 0
x = 4
x + 10 = 0
x = -10
The solutions to this equation are 4 and -10
Hope this helped!
~Just a girl in love with Shawn Mendes