You read that wrong.  It should be read like this:

Solve this equation for the variable y:

5x + 4y = 12
                    Get the y-term by itself on the left by
                    adding -5x to both sides:

     4y = -5x + 12
                    Divide every number by the coefficient of y,
                    which is 4:

     y = x + 

                y = x + 3

                y = x + 3

Rule:

For the equation of line

               y = mx + b

the slope = m and the y-intercept is the point (0,b)

Compare

                y = x + 3

with            y = mx + b

and you see that m =  and b = 3

so the slope = m =  and the

y-intercept = (0,b) = (0,3) 

--------------------------------

What is the corresponding value of y when x=16.

Substitute 16 for x in

                y = x + 3

                y = (16) + 3

                y = -20 + 3

                y = -17

Edwin