The slope intercept form of an equation is written y = mx + b where m is the slope and b is the y-intercept (value of y when x = 0).
I am assuming that you understand that the equation is y = -3x + b, but don't know how to find b.
Two ways to solve this problem are algebraically and graphing. To solve the problem by graphing, plot the point (8,0), then move up 3 left 1 until you reach the x-axis (x=0).
To solve the problem algebraically, substitute the given x and y values for x and y in the equation for slope-intercept and solve for b.
y = -3x + b
0 = -3 * 8 + b
0 = -24 + b
b = 24
The solution is y = -3x + 24