Disp. with Constant Accel. Formula Help

[shyboyswin]

Homework Statement

A jet plane lands with a velocity of +100 m/s and can accelerate at a maximum rate of -5.0 m/s² as it comes to rest. Can this plane land at an airport where the runway is 800 meters long?

v_i = +100 m/s
a = -5.0 m/s^2

Homework Equations

The only equations in the chapter are as follows, but I don't think either can help me:
a_{avg} = \frac{\Delta x}{\Delta t}
\Delta x = \frac{1}{2}(v_i +v_f)\Delta t

The Attempt at a Solution

I have attempted the following, but the answer does not seem to make any sense to me, since I need a distance and not a time.

\frac{+100 m/s}{-5.0 m/s^2} = -20 s

 

[PhanthomJay]

Homework Statement

A jet plane lands with a velocity of +100 m/s and can accelerate at a maximum rate of -5.0 m/s² as it comes to rest. Can this plane land at an airport where the runway is 800 meters long?

v_i = +100 m/s
a = -5.0 m/s^2

Homework Equations

The only equations in the chapter are as follows, but I don't think either can help me:
a_{avg} = \frac{\Delta x}{\Delta t}
\Delta x = \frac{1}{2}(v_i +v_f)\Delta t

The Attempt at a Solution

I have attempted the following, but the answer does not seem to make any sense to me, since I need a distance and not a time.

\frac{+100 m/s}{-5.0 m/s^2} = -20 s

Shyboyswin, welcome to PF! Your first equation is not correct, looks like you may have made a typo error, because you have used the correct magnitude values in your calculation for time, although you're messing up your plus and minus signs. Once you get the time, use your 2nd equation to calculate the required stopping distance.