a + b + c = 180 for b

[math_celebrity]

a + b + c = 180 for b

We have a literal equation. Subtract (a + c) from each side of the equation to isolate b:
a + b + c - (a + c) = 180 - (a + c)

The (a + c) cancels on the left side, so we have:
b = 180 - (a + c)

or, distributing the negative sign:

b = 180 - a - c