This equation is in slope-intercept form, it is written as y = mx + b where m is the slope and b is the y-intercept. This means 3 is the slope and -10 is the y-intercept.
It should be 7.97 decigrams, since you want to convert 3 milligrams to decigrams (0.03), it is asking "How much more" which means you subtract. 8 subtracted from 0.03 is 7.97.