Hey there!
The answer to your question is a = s - b - C/2
To solve for "a", we must get "a" on it's own side of the equal sign. We can first subtract "s" from both sides:
0 = a + b + C/2 - s
Then, we can subtract a from both sides:
-a = b + C/2 - s
Finally, we divide both sides by -1, so a is by itself.
a = -b - C/2 + s
And we can rearrange it to the standard form:
a = s - b - C/2
Good luck, and hope it helped! Have a great day!