Question is poorly worded. Should read, "find the length of the line segment connecting the two dots, in terms of a, b, c and d."
Use the Pyth. Theorem:
(change in x)^2 + (change in y)^2 = (segment length)^2
Then:
(segment length)^2 = (d-b)^2 + (c-a)^2
and (segment length) = sqrt [ (d-b)^2 + (c-a)^2 ]
