Should be 18x^4-12x.
Let me know if you want more of an explanation.
Answer:
k = 12
Step-by-step explanation:
Given:
The equation 
To find:
Value of
for which the given equation has one distinct real solution.
Solution:
The given equation is a quadratic equation.
There are always two solutions of a quadratic equation.
For the equation:
to have one distinct solution:

Here,
a = 2,
b = -k and
c = 18
Putting the values, we get:

The equation becomes:

And the one root is:

Speed = distance / time
30 = d / 2.5
30 * 2.5 = d
75 = d
40 = d / 1.875
40 * 1.875 = d
75 = d
50 = d / 1.5
50 * 1.5 = d
75 = d
60 = d / 1.25
60 * 1.25 = d
75 = d
24 = 75 / time
time = 75/24
time = 3.125 hours