Question 125734
The first part of your working is correct, until -3/10-1/8.

-3/10-1/8
= -12/40-5/40 (both numerator & denominator is multiplied by 4 for -3/10 and 5 for -1/8)
= -17/40 (there is still a minus sign in front of 12/40 so another way to see it is 0-12/40-5/40=-17/40 and not just 12/40-5/40=7/40)

=) hope this answers your query.