Which construction is illustrated

Which is an equation for the nth terms of the sequence 12,15,18,21

What is the value of n in the equation -1/2(2n+4)+6=-9+4(2n+1)

Andy wants to find the distance d across a river. he located points as shown in the figure, then use similar triangles to find that d = 223.5 feet. how can you tell without calculating that he must be wrong? tell what you think he did wrong and correct his error. round your answer to the nearest tenth if necessary. andy must have used the wrong . the distance d is actually feet.