You need to find the width of the stream. you cannot measure the width directly, so you draw a diagram showing lengths that you can measure. your diagram contains two similar triangles, one in the stream and one on land. use the diagram to find the width of the stream.

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graphically?   well, you graph them both, the solutions is where they both intersect each other, since there are two solutions, meaning they will meet twice, or namely the linear will run into the quadratic curve twice, and where they meet, those x,y coordinates are the solutions to the system.