In addition, if you would like to use the function subset(...) from base R.
cityincdiff <- data.frame(zip=c('000650','990112'),
total_a=c(15,16),
Difference=c(2,17777),
City=c('Philly','Newyork'),
stringsAsFactors = FALSE)
subset(x = cityincdiff, subset = Difference==17777, select = 'City')
#> City
#> 2 Newyork
subset(x = cityincdiff, subset = Difference==17777, select = 'City',
drop = TRUE)
#> [1] "Newyork"
Created on 2019-03-20 by the reprex package (v0.2.1)
Best regards
Adam