The distance between two points \( (x_1, y_1) \) and \( (x_2, y_2) \) is given by the distance formula:
\[
d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}.
\]
Substitute the given points \( (2, y) \) and \( (10, 3) \) and the distance \( d = 10 \):
\[
10 = \sqrt{(10 - 2)^2 + (3 - y)^2}.
\]
Simplify:
\[
10 = \sqrt{8^2 + (3 - y)^2} \quad \implies \quad 10 = \sqrt{64 + (3 - y)^2}.
\]
Square both sides:
\[
100 = 64 + (3 - y)^2.
\]
Subtract 64 from both sides:
\[
36 = (3 - y)^2.
\]
Take the square root of both sides:
\[
\pm 6 = 3 - y.
\]
Step 1: Solve for \( y \).
1. \( 6 = 3 - y \) gives \( y = -3 \).
2. \( -6 = 3 - y \) gives \( y = 9 \).
Thus, the possible values of \( y \) are \( y = -3 \) and \( y = 9 \).
Conclusion:
The value of \( y \) is either \( -3 \) or \( 9 \).