[tex]\displaystyle\bf\\x^{2} -4x+b=0\\\\2x_{1} +3x_{2} =5\\\\\\Teorema \ Vieta \ :\\\\x_{1} + x_{2} =4\\\\\\\left \{ {{2x_{1} +3x_{2} =5} \atop {x_{1} +x_{2} =4 \ |\cdot(-2)}} \right. \\\\\\+\left \{ {{2x_{1} +3x_{2} =5} \atop {-2x_{1} -2x_{2} =-8}} \right. \\-----------\\x_{2} =-3\\\\x_{1} =4-x_{2} =4-(-3)=4+3=7\\\\Teorema \ Vieta \ : \ \\\\b=x_{1} \cdot x_{2} =7\cdot(-3)=-21\\\\\\Otvet \ : \ b=-21[/tex]