Answer:
Disappointed by not being able to picnic by the river, they went home sadly.
Step-by-step explanation:
A past participle refers to a verb in its third form. They could also serve as adjecive They (most past participles) are usually characterized with the suffix "-ed" or "-d", "-en", "-n" & occasionally, "-t". It is also important to note that a past participle signifies an action which has been accomplished or brought to completion. Some examples of verbs and their past participles are shown below:
I. walk - walked
II. eat - eaten
III. go - gone
For the two sentences to be joined by a past participle, the verb disappointed changes from the past tense form to the past participle form. Hence, the sentences change from "They were disappointed by not being able to picnic by the river. They went home sadly." to "Disappointed by not being able to picnic by the river, they went home sadly."