If i have two lines, will they cross? explain fully. what postulate me answer this question?

What is the axis of symmetry of the function f(x)=-(x+ 9)(x-21)

Which of the following is best modeled using a linear equation y=ax+b, where a is less than 0?

Multiply and simplify. 2x^2 y^3 z^2 · 4xy^4 x^2 i will mark you brainliest if right. show how you got the answer, !