Answer:
Since (2x, x + 17, x + 21, ...) is an arithmetic sequence, we know that the common difference between consecutive terms is constant.
The common difference can be found by subtracting any two consecutive terms in the sequence. Let's subtract the second term (x + 17) from the first term (2x):
2x - (x + 17) = x - 17
Now let's subtract the third term (x + 21) from the second term (x + 17):
(x + 17) - (x + 21) = -4
Since the common difference is constant, we can set the two expressions for the common difference equal to each other:
x - 17 = -4
Solving for x, we get:
x = 13
Therefore, the value of x that makes the sequence (2x, x + 17, x + 21, ...) an arithmetic sequence is x = 13.