What are the roots of the equation x^2+3x-18=0

First you open 2 parenthesis with x in each one:

(x) (x) = 0

Then, you place the sign of the second term (the x term) in the first parenthesis.

(x+) (x) = 0

Next, you multiply the signs of the x term and the sign of the constant (the third term).

Since + * - = -, then you place a -.

(x+) (x-) = 0

You look at the constant (-18) and the second term (3). What two numbers multiplied give you - 18 but subtracted (since the signs inside the parenthesis are opposite) give you 3? Those numbers are 6 and 3.

Since the result of the substraction need to be 3, you place the 6 with the + sign and the 3 with the negative sign (if you think of it, 6-3=3 but 3-6 = - 3. That's the reason behind placing the numbers)

(x+6) (x-3) = 0

Therefore,

x+6=0 or x-3=0

x=-6 or x=3

Your roots are - 6 and 3.