What is the value of the expression 3y-z over y+5z

Which matrix can be multiplied to the left of a vector matrix to get a new vector matrix ?

Achemist has 200 ml of a 10% sucrose solution. she adds x ml of a 40% sucrose solution. the percent concentration, y, of the final mixture is given by the rational function: the chemist needs the concentration of the final mixture to be 30%. how many milliliters of the 40% solution should she add to the 10% solution to get this concentration?

How do you use the elimination method for this question? explain, because i really want to understand!