Copper telephone wire has essentially no sag between poles 35.0 m apart on a winter

day when the temperature is -20.0℃. What is the length of the wire on a summer day
when the temperature is 35.0℃?

1 answer

delta L = alpha*L*delta T
is the formula to use. I could not find a consistent value for alpha on the web. I would substitute 35 meters for L, delta T will be 35 - (-20) = 55 and use values in your text/notes for alpha. Post your work if you get stuck.