Answers: a = 49
b = 10
c = 105
Answer: 
Step-by-step explanation:
Let be "x" the time in minutes a 150-pound person must walk at 4 mph to use at least 190 calories.
The amount of calories that a 150-pound person uses in 1 minute when walking at a speed of 4 mph is:

Therefore, knowing this, we can write the following proportion:

Finally, we must solve for "x" in order to find its value.
Multiplying both sides of the equation by 190, we get this result:

K^2-5k-50
(k-10)(k+5)
This is because k*k=k^2 (your first term), 5k-10k=-5k (your second term) and -10*5=-50 (your last term).
Distribute to check.