-12(5/6a-7/8)+1/14(3 1/2a-1 2/5)

-12(5/6 a -7/8) + 1/14(3 1/2 a- 1 2/5)

First change mixed fractions to be full fractions

-12(5/6 a -7/8) + 1/14 (7/2 a - 7/5)

Now multiply out

-12(5)/6 a -12(-7)/8) + 7/14(2) a -7/14(5)
-60/6 a + 84/8 + 7/28 a -7/70
-10a + 10.5 + 1/4a - 1/10
-10a + 0.25a +10.5 -0.1
-9.75a + 10.4

*if this =0 and you need to solve for a then
9.75a=10.4
a= 10.4/9.75
a=16/15

31/6= 19/6 19/6 *1/7=19/42