Underspecified, as you appear to be using odd definitions of one or more terms. The straightforward interpretation of consecutive_differences (a, b, c, ... y, z) would be ((a - b), (b - c), ... (y - z)). Parsing the phrase "sum of consecutive differences of (2, 4, 5, 7)" as "sum (consecutive_differences (2, 4, 5, 6)))" and evaluating, one gets

sum (consecutive_differences(2, 4, 5, 7))
        = {definition of consecutive_differences}
sum((2 - 4), (4 - 5), (5 - 7))
        = {evaluate sub-expressions}
sum(-2, -1, -2)
        = {evaluate sum}
-5
[download]

but recognizing sum(a, b, c, ... z) as (a + b + c + ... + z), the above simplifies algebraically to

sum (consecutive_differences(2, 4, 5, 7))
        = {definition of consecutive differences}
sum((2 - 4), (4 - 5), (5 - 7))
        = {definition of sum}
((2 - 4) + (4 - 5) + (5 - 7))
        = {subtraction equals addition of negative}
(2 + -4 + 4 + -5 + 5 + -7)
        = {associativity and symmetry of addition}
(2 + (4 - 4) + (5 - 5) - 7)
        = {removal of zero terms}
(2 - 7)
        = {arithmetic}
-5
[download]

so that the "sum of consecutive differences" is the first minus the last.