What expression would make a zero pair with 5x?

How do i do this activity, is appreciated

The swimming pool is open when the high temperature is higher than 20∘c. lainey tried to swim on monday and thursday (which was 3 days later). the pool was open on monday, but it was closed on thursday. the high temperature was 30∘c on monday, but decreased at a constant rate in the next 3 days. write an inequality to determine the rate of temperature decrease in degrees celsius per day, d, from monday to thursday.

Need 50 will mark brainliest answer. find the lengths of all the sides and the measures of the angles.