Someone
which sentence from the paragraph uses a problem-solution structure?

"then two massive fires flared up within two blocks of each other in the western addition section of the city."

"the fire in the mission district was subdued with water found blocks away in a hydrant."

"the main reservoirs of water for the city were 20 miles away, and some of the pipelines to them had been destroyed because they were constructed along the quake’s fault line."

"at one point it became a three-mile-wide wall of raging flames with billowing black clouds of smoke."

"The fire in the Mission District was subdued with water found blocks away in a hydrant." is the sentence which uses a problem-solution structure.

In a sentence containing a problem-solution structure, you should be able to identify:

What could help is to look for a "pivot verb" which serves as the transition from the problem to the solution. These are usually action verbs, like subdue in the text. Other examples could be fix, address, solve, respond to, etc.

put the trash into the back of the truck and drive the truck to the back. then you must unload the truck.