Answer:
After using colons and semicolons correctly the sentence looks like this:
Cesar Chavez understood the plight of Mexican American farmworkers; after all, he had been one himself.
Step-by-step explanation:
Colons and semicolons are punctuation elements that are used to continuation or union among sentences. Colons in the first place show that something is a consequence of the previous sentence. In our case, we want to express that the fact that made Cesar Chavez understand the plight of Mexican American farmworkers was that he was one of them. So it is the element we provide as evidence, or example and therefore colon is employed. On the other hand, semicolons combine dependent and independent clauses, and the dependent clause here is that he for being one of the Mexican American Farmworkers knew the plight of them. It could be another sentence by itself but it provides more detail on the initial sentence to complement it.