So Jim runs at most 37 miles, but no more than that. 37 miles is the maximum value here.
To use an inequality, remember the alligator eats the bigger number (yes, I still use this metaphor to understand inequality signs)
If we say that T represents the total number of miles, we can write the expression as
T ≤ 37
keep in mind, that is a "less than or equal to" symbol, since "37 miles" is included in the possible total.
So if the amount of running completed so far is 19, the final total for the week, T, will look like
T = 19 + t
where, as stated in the question, t is the number of additional miles
Combine the two:
19 + t ≤ 37
Subtract 19 from both sides to isolate t, and there's your answer.