The usual margin of error we strive for is a 95% probability that our estimate is within the margin. Assuming a normal distribution (and that we have a big enough sample not to worry about t tests) that corresponds to between minus and plus two standard deviations of the mean (remember the 68-95-99.7 rule). So our margin of error is plus or minus two standard deviations.
Look for the y-intercept since we know the slope is 3 we just going to plug in -5=3(1)+y -5-3=y so the y-intercept is 8 now you can find the equation y=3x-8