Concept:
Image of a point in a plane is found using reflection formula.
Step 1: Given point:
\[
P = (1,0,3)
\]
Plane:
\[
x + y + z = 1
\]
Step 2: Use reflection formula:
\[
\text{Image} = P - 2\frac{ax_1 + by_1 + cz_1 - d}{a^2+b^2+c^2}(a,b,c)
\]
Substitute:
\[
= (1,0,3) - 2\frac{1+0+3-1}{3}(1,1,1)
\]
\[
= (1,0,3) - 2(1,1,1)
= (1,-2,1)
\]
Final Answer:
\[
\hat{i} - 2\hat{j} + \hat{k}
\]