Possible coefficient of x

The following shows a question of a mathematics quiz:rewrite 3x²+■x-8 as (3x+m)(x+n),where m and n are integers. 
However, due to a printing error, the coefficient of x is covered by an ink drop and cannot be seen. Write down the possible coefficients of x.explain your answer.

M,n are factors of 8 with mn=-8
let b = Coefficient of x 
b = m+3n
Possible (m,n,b) : (1,-8,-23),(2,-4,-10),(4,-2,-2),(8,-1,5),(-1,8,23),(-2,4,10),(-4,2,2),(-8,1,-5)

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