Subject:Mathematics ​

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

Serena is making an experiment. for that, she needs 20 grams of a 52% solution of salt. she has two large bottles of salt water: one with 40% and the other with 70% of salt in them. how much of each must she use to make the solution she needs? i am in need of

Why is x= 4 a solution to the proportion 14/x 56/1 6