Translation:Let's go to the theater.
-(으)ㅂ시다 is actually in 하오체 (the second high formal formality level) which this course does not cover, but it is quite commonly used whereas -십시다, the corresponding propositive ending in 합쇼체, is rarely used and doesn't even sound very polite.
The definition of 극장 is "a building or a facility with a stage and seats where (1) plays, music, dance, and other performances take place or (2) movies are shown". A movie theater is, however, usually called 영화관, so 극장 usually refers to (1).
When I answered with "Let's go to the theater" I got credit for it but I was "corrected" and it said I had an "extra space".
I get why in teaching english to koreans you would want to make them think about contractions, but as english speakers im not sure you need to separate the 's from let.
We can also say.. 극장에 가자 which is ways easier... I learnt it from a youtube channel "conversational korean" a really good teaching method!