Answer:
y=-x+4
The equation is in the form y=mx+c, where m is the slope and c is the y-intercept, so we know that the slope (m) is -1, and the y-intercept (c) is 4.
The y-intercept is the value of y when x=0. So you can mark (0,4) as a point on the line.
The next thing I would do is take any value for x and find another point on the line. Let's let x=2:
y=-2+4
y=2
So now we know another point on the line (2,2). Mark this on the graph too.
Then connect these two points and extend to the line to finish off your graph, and you're done!
(PS: if the whole 'y-intercept' thing gives you a headache, you can just pick any two values for x, sub them in to find the points, mark those points and connect. There's also a method for graphing using just the slope and y-intercept, but the method above is usually what people prefer to do)
I've attached a sketch that corresponds to my answer to give you an idea:
