a=((b[1]+b[2])*.5)*h

Solution for a=((b[1]+b[2])*.5)*h equation:

Simplifying
a = ((b[1] + b[2]) * 0.5) * h

Reorder the terms for easier multiplication:
a = ((1b + b[2]) * 0.5) * h

Reorder the terms for easier multiplication:
a = ((1b + 2b) * 0.5) * h

Combine like terms: 1b + 2b = 3b
a = ((3b) * 0.5) * h

Remove parenthesis around (3b)
a = (3b * 0.5) * h

Reorder the terms for easier multiplication:
a = (3 * 0.5b) * h

Multiply 3 * 0.5
a = (1.5b) * h

Remove parenthesis around (1.5b)
a = 1.5b * h

Multiply b * h
a = 1.5bh

Solving
a = 1.5bh

Solving for variable 'a'.

Move all terms containing a to the left, all other terms to the right.

Simplifying
a = 1.5bh