Question 1083108

let's a number be {{{x}}}
a number multiplied by {{{-9}}}: {{{-9x}}}

the sum of {{{13}}} and three times the number:{{{13+3x}}}

if {{{-9x}}} subtracted from the {{{13+3x}}}, we have:

{{{13+3x-(-9x)}}}

={{{13+3x+9x}}}

={{{13+12x}}}