for every value of x you put in, you can only get 1 y value exg if you subsitue 5 for x then then you will get only 4 and only 4
but it is possible to have 5 for x and get 4 and 20 for x and also get 4,
one easy way is to use the vertical line test
if you graph the equation and you draw a vertical line throught the line, the vertical line should pass through the equation line only once
if we manipulate the equation, we would see that is is = y=x/4+5/4 which is a linear equation that is a striaght line and this passes the vertical line test
