Answer: y = 21x + -105 or y = 21x - 105
Explanation:
Our equation is going to be in the format of y = mx+b, where m is the slope, and b is the y-intercept.
For this, you can find the slope by doing
(y₁-y₂)÷(x₁-x₂)
So we just sub in one of the x intercept points and the point that we already have, so let's do (-21-0)/(4-5). That is equal to 21, therefore, 21 is our slope.
So we have y = 21x + b, so now we can sub in one of our points for y and x in the equation. Let's do (4,-21).
-21 = 21(4) + b
-21 = 84 + b
-105 = b
So our equation is y = 21x + -105, meaning we have a slope of 21 and a y intercept of -105.
Edit: you can also do y = 21x - 105