Answer :

You can subtract 78.2 from 287 to get the x value alone. Your new expression will look like this:
.5x=208.8
Next you divide by .5 (or multiply the reciprocal 1/.5 depending on how you were taught)
x=417.6

Hope that helps!!
0.5x + 78.2=287
           -78.2 -78.2
0.5x         = 208.8
/0.5             /0.5
x              =417.6