Answer
As given
The weight y of an object on Jupiter is proportional to the weight x of the object on Earth.
i.e

y = kx
where k is the constant of proportionality.
An object that weighs 150 pounds on Earth would weigh 379.2 pounds on Jupiter.
put in the eqaution
y = kx
379.2 = k × 150

k = 2.528
As given
A rock that weighs 12.64 pounds on Jupiter .
put in the equation
y = kx
12.64 = 2.528 × x

x = 5 pounds
Thus
A rock that weighs 12.64 pounds on Jupiter would weigh 5 pounds on Earth.
Therefore
y = A rock that weighs 12.64 pounds on Jupiter would weigh 5 pounds on Earth.