Question

p=a+b+2c
a=

Answer 1

p = a + b + 2c
p - b = a + 2c
a = p - b - 2c

Answer 2

p = a + b + 2c add -p to both sides of the equation
p - p = a+b+2c-p
0=a+b+2c-p add-a to both sides of the equation
-a=a-a+b+2c-p
-a=0+b+2c-p multiply by (-1) both sides of the equation
(-1)(-a)=(-1)(b+2c-p)
a=-b-2c+p rearrange and you get:
a=p-b-2c