Explanation:
 
let's say one integer is X .... 
so , Another one is (X -4)..... 
 so , X × (X - 4) = 45 
 x² - 4x - 45 = 0 
 x² - 9x + 5x - 45 = 0 
 x(x - 9) +5(x - 9) = 0 
 (x-9) × (X+5) = 0 
 
 x-9 = 0 OR x+5 = 0 
 X= 9 X= -5 
 X is not a negative integer ......
 so , X = 9 
 X-4 = 5 .......