All you have to do here is to use the fact that
#color(blue)(ul(color(black)("1 dag = 10 g")))#
In other words, you need
To convert your sample to grams, set up this conversion factor by adding decagrams as the denominator and grams as the numerator.
You will end up with
#0.785 color(red)(cancel(color(black)("dag"))) * "10 g"/(1color(red)(cancel(color(black)("dag")))) = color(darkgreen)(ul(color(black)("7.85 g")))#
The answer must retain the number of sig figs you have for your measurement.