Speed and direction

Driving along a crowded freeway, you notice that it takes a time t to go from one mile marker to the next. When you increase your speed by 4.0 mi/h, the time to go one mile decreases by 12 s. What was your original speed?
....... mi/h ?

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...speed = x + 4 mi/h

New time = distance/new speed = 1/(x + 4) hours

By data, 1/x - 1/(x + ...