Chapter P - Prerequisites: Fundamental Concepts of Algebra - Exercise Set P.4 - Page 61: 35

$25-49x^2$ Note that standard form would list the variables first in decreasing order of exponent, so another way to write this answer is $-49x^2+25$.

$$(5-7x)(5+7x)$$ $$=(5\times 5)+(5\times 7x)+(-7x\times 5)+(-7x\times 7x)$$ $$=25+35x-35x-49x^2$$ $$=25-49x^2$$ Note that standard form would list the variables first in decreasing order of exponent, so another way to write this answer is: $$-49x^2+25$$