Answer
Fill the blank with $10.$
Work Step by Step
With the foil method, with m and n being integers, we find that
$(x+m)(x+n)=x^{2}+(m+n)x+ mn,$
so ,the sum of mn is the coefficient of x, and
m and n are factors of the constant term.
Reversing, when we want to factor $x^{2}+bx+c,$ we search for two integers, m and n such that
their sum is b,
their product is c.
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Here, we are given $b=13, c=30$ , and one of the integers is m=3.
The other must be n=10, since
$3\cdot 10=30,\quad 3+10=13.$