Ihave an equation that is about combining terms and i am looking at how to check your work and the example is (image included) and i don't get how they got 12, is it always 12?

yes because there is exactly one y value for every x value

for function there can't be more than one value for every x value

b, i believe, but not sure.

the domain of a function is the set of input values, x , for which a function is defined. the domain is shown in the left oval in the picture below. the function provides an output value, f(x) , for each member of the domain.

twenty-six miles over two gallons because the proportion for the original 13 miles to one gallon would be written 13/1 and, doing the same thing to the top and bottom, i multiplied by 2 and got 26/2