He was traveling 63 mph
divide 315 by 5 and you get 63
check your answer by multiplying his speed (63mph) by 5 (number of hours)
You have forgot to attach the picture
You just need to plug those expressions inside the formula: it doesn't matter if they're expressions involving a variable instead of plain numbers: the formula becomes

If you want, you can simplify it by expanding the square and then multiply the two parenthesis:

N=0 bc you remove the parentheses, cancel the equal terms what would be 6 being cancel it would look like -24n=3n now and move the variable to the left collet like terms and then you will divide the sides. Your answer is 0
Point Slope Formula: y-y₁=m(x-x₁)
Linear Standard Form: Ax+By=C
Slope Intercept: y=mx+b