How do you find what the M in equations like this one and how do I answer this equation.

The solution, or root, of an equation is any value or set of values that can be substituted into the equation to make it a true statement. For the first example, the solution for x is 13. The second example has two values that will make the statement true, namely 2 and –2

this is a direct variation proportion. y = kx

y is the total amount due, k is the constant of variation, and x is the number of texts sent.

plug in the 3.20 and the 16 to solve for the "k". this is the constant of proportionality. it is the value that will not change.

3.20 = k(16). solving you get $.20 per text.

the equation would then be y = .20x