In order to factor  , first we need to ask ourselves: What two numbers multiply to 20 and add to 9? Lets find out by listing all of the possible factors of 20
Factors:
1,2,4,5,10,20,
-1,-2,-4,-5,-10,-20,List the negative factors as well. This will allow us to find all possible combinations
These factors pair up to multiply to 20.
1*20=20
2*10=20
4*5=20
(-1)*(-20)=20
(-2)*(-10)=20
(-4)*(-5)=20
note: remember two negative numbers multiplied together make a positive number
Now which of these pairs add to 9? Lets make a table of all of the pairs of factors we multiplied and see which two numbers add to 9
| First Number | | | Second Number | | | Sum | | 1 | | | 20 | || | 1+20=21 | | 2 | | | 10 | || | 2+10=12 | | 4 | | | 5 | || | 4+5=9 | | -1 | | | -20 | || | -1+(-20)=-21 | | -2 | | | -10 | || | -2+(-10)=-12 | | -4 | | | -5 | || | -4+(-5)=-9 | We can see from the table that 4 and 5 add to 9. So the two numbers that multiply to 20 and add to 9 are: 4 and 5
Now we substitute these numbers into a and b of the general equation of a product of linear factors which is:
 substitute a=4 and b=5
So the equation becomes:
(x+4)(x+5)
Notice that if we foil (x+4)(x+5) we get the quadratic again
So the quadratic  factors to
Set each factor equal to zero
So our answer is
or
Answer by chitra(359)  (Show Source):
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