скорее всего у тебя опечатка и у тебя не 2x, а x^2
х^2 + 5b – 14 = ( х + 7) ( х – b)
A=1 B=5 C=-140


по теореме Виета:
b=2" alt="\left \{ {{x_{1}+x_{2}=-B} \atop {x_{1}*x_{2}=C}} \right. \left \{ {{-7+b=-5} \atop {-7*b=-14}} \right. => b=2" align="absmiddle" class="latex-formula">
Ответ: 2