y = mx + b, where m = slope, and b = y-intercept.
Since you are not given the y-intercept, you have to solve for it through the equation, y = -3x (which is the slope you are given) + b, and replace x and y with values given, which are (1,4)
y = -3x + b
4 = -3(1) + b
4 = -3 + b
7 = b
Then replace b in the first equation to get the answer
y = -3x + 7
