Concept:
The equation of a straight line passing through two points can be found using slope formula and point-slope form:
\[
m = \frac{y_2 - y_1}{x_2 - x_1}, \quad y - y_1 = m(x - x_1)
\]
Step 1: Find slope
Given points:
\[
(1,2), \quad (-1,4)
\]
\[
m = \frac{4 - 2}{-1 - 1} = \frac{2}{-2} = -1
\]
Step 2: Use point-slope form
Using point \((1,2)\):
\[
y - 2 = -1(x - 1)
\]
\[
y - 2 = -x + 1
\]
Step 3: Rearrange
\[
x + y = 3
\]
Thus, the required equation is:
\[
\boxed{x + y = 3}
\]