Amountaineer climbed 1,000 feet at a rate of x feet per hour. he climbed an additional 5,000 feet at a different rate. this rate was 10 feet per hour less than twice the first rate. which expression represents the number of hours the mountaineer climbed

Your question asks for an expression, so you would expect an answer in the form of an equation containing variables. This is because you have only 2 independent equations with 3 unknowns. That makes the system unsolvable. 

Suppose v₁ is the mountaineer's velocity during the 1,000-foot trail. Then, v₂ is his speed for the 5,000-foot trail. The equation relating these two variables is:

v₂ = 2v₁ - 10 ---> equation 1

Then, you find the equation for the total number of hours he climbed. That would be the distance divided by their respective speed.

t = 1000/v₁ + 5000/v₂ > equation 2

Substituting equation 1 to equation 2,

t = 1000/v₁ + 5000/(2v₁ - 10)
t = 1000/v₁ + 5000/2(v₁-5)
t = 1,000/v₁ + 2,500/(v₁-5)