Hey there,
solving for a is just like you think of all other letter as a constant
i.e the only variable is a
so you need to subtract b as a start
d=a+b-c/3
-b -b
=============
-b+d=a-c/3 now it's time to add c/3 (isolate a by itself in one side)
-b+d=a-c/3
+c/3 +c/3
==============
c/3-b+d=a
Hence you have solved for a
