(a) How far from the stop line do you pass the Porsche?
At the instant you pass the Porsche, the x values of your car and the Porsche must be equal. Let's write the general equations for x(t):
General equation: x = xo + vo t + ½ a t2
For your car : xyou = -15 + 11.11 t + 0
For the Porsche : xPorsche = 0 + 0 +½ (3) t2 = 1.5 t2
Set xyou = xPorsche, and solve for the time(s) when the two cars are at the same position. Then plug the time back into the x equation(s) to find the position.
xyou = xPorsche leads to: -15 + 11.11 t = 1.5 t2
Bringing everything to one side gives: 1.5 t2 - 11.11 t + 15 = 0
Solve using the quadratic formula:
where a = 1.5, b = -11.11, and c = 15
This gives two values for t, t = 1.776 s and t = 5.631 s.
To answer question (a), plug t = 1.776 into either of your x expressions. Both equations must give the same value. This serves as a useful check!
For you, when t = 1.776 s, xyou = 4.73 m.
For the Porsche, when t = 1.776 s, xPorsche = 4.73 m.