# In that day

"At that time" or "Then"

# Tyre will be forgotten for seventy years

"for seventy years it will be like the people have forgotten about Tyre"

# like the days of a king

"like the years of a king" or "which is about as long as a king lives"

# like in the song of the prostitute

This speaks about the people of Tyre as if they were a prostitute.