плиз
[tex]2(k - 7)(k + 5) - (k - 5) {}^{2} - (k - 7)(7 + k) \: pri \: k = - \frac{1}{2} [/tex]

2(k-7)(k+5)-(k-5)²-(k-7)(7+k)= 2(k²-2k-35)-(k²+25-10k)-(k²-49)= 2k²-4k-70-k²-25+10k-k²+49= 6k-46= 6*(-1/2)-46= -3-46= -49