f(x)=(x+3)(x+1)

Solution for f(x)=(x+3)(x+1) equation:

Simplifying
f(x) = (x + 3)(x + 1)

Multiply f * x
fx = (x + 3)(x + 1)

Reorder the terms:
fx = (3 + x)(x + 1)

Reorder the terms:
fx = (3 + x)(1 + x)

Multiply (3 + x) * (1 + x)
fx = (3(1 + x) + x(1 + x))
fx = ((1 * 3 + x * 3) + x(1 + x))
fx = ((3 + 3x) + x(1 + x))
fx = (3 + 3x + (1 * x + x * x))
fx = (3 + 3x + (1x + x2))

Combine like terms: 3x + 1x = 4x
fx = (3 + 4x + x2)

Solving
fx = 3 + 4x + x2

Solving for variable 'f'.

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

Divide each side by 'x'.
f = 3x-1 + 4 + x

Simplifying
f = 3x-1 + 4 + x

Reorder the terms:
f = 4 + 3x-1 + x