Help me pleaseeeeeeeeeeeeeeeeeeeeee

Answer:
A
Step-by-step explanation:
[tex]s=\frac{a+b+c}{2}[/tex]
multiply both sides by 2
[tex]2s = a + b +c[/tex]
substract b and c from both sides to get a by itself
[tex]a = 2s - (b + c)\\a = 2s - b - c[/tex]
so the answer is A.