Find two factors of 18 with the sum of 19

A*b=18 [1]
a+b=19[2]

[2] a=19-b ___[3]

put [3] in [1]
(19-b)*b = 18
19b-b^2 = 18
b^2-19b+18 = 0
(b-18)(b-1) = 0
so b=18 or b=1

if b=18 then a=19-18=1
if b=1 then a=19-1=18

SO two factors are 1 and 18