What are the steps to solve 13 and 15?

f (x) which is said " f of x " is essentially the same as saying Y

you would plug in random numbers for x to see what y would come out too. I would use numbers that I know would come out with a whole number instead of a fraction. for instance with f (x) =3/4x+9 if I used 2 for x then I would get something like this. f(x)=3/4 * 2/1 +9    f(x)=6/4 +9   f(x)=10 1/2

Instead I would use numbers that would come out even like x=4 because I know that 4x3=12 and 12 is divisible by 4 evenly. I would then get f(x)=12

You can plug in as many points for x as you want to get its y coordinate.  Hope that helps

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