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