Here's why:
* Passive Voice Structure: The sentence follows the passive voice structure: Subject + be verb + past participle + by + agent (optional)
* Subject: "The president" is the subject of the sentence.
* Be Verb: "was trying" is the be verb.
* Past Participle: "develop" is the past participle of the verb.
* Agent: The agent, the one doing the action, is missing. We don't know who or what is being developed.
To change the sentence to the active voice, we need to identify the agent and make it the subject:
* Active Voice: Someone (or something) was trying to develop the president's economy.
This sentence is in the active voice because the subject ("Someone") is the one performing the action.