Calculate the magnitude of the horizontal vector that is 100 units long and is oriented at 45 degrees

Your question is somewhat twisted up.  We would be justified in simply asking you to correct the question.  But I think I know what you're talking about, so I'll try to answer it.

A horizontal vector is one that's oriented at an angle of zero degrees, not 45 degrees.  I think what you want is the horizontal component of the vector that you're describing.

Its horizontal component is  100 cos(45)  = 100 / √2  =  70.71 units.

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.