First, look at the points where the x-axis and y-axis are crossed by the line. They are: (0,3) and (3/2,0). From these two points, you can get the slope of the line shown: it is the change in y divided by the change in x. Looking at the two points already identified, the change in y (aka the rise) is 3 - 0 = 3 and the change in x (aka the run) is -3/2, so the slope of the line is rise/run = m = 3/(-3/2) = -2.
One equation of a straight line is y = m*x + b. From the last step, we have m = -2. Now we need to determine b. When x = 0, we can tell from the graph that y = 3, so that 3 = -2*0 + b. Simplifying b = 3. So, plugging in b into y = -2*x + b, the best (simplest) linear equation describing the relationship between x and y is: y = -2*x + 3.