To find the unknown side length of a right triangle using the Pythagorean Theorem, we use the formula:
\[ a^2 + b^2 = c^2 \]
where \( c \) is the length of the hypotenuse, and \( a \) and \( b \) are the lengths of the other two sides.
In this triangle, we have two known sides: \( a = 15 \) and \( b = 3 \). We need to find the length of the hypotenuse \( c \).
Plugging in the values we have:
\[ 15^2 + 3^2 = c^2 \]
Calculating the squares:
\[ 225 + 9 = c^2 \]
Adding these values:
\[ 234 = c^2 \]
Now, to find \( c \), we take the square root of both sides:
\[ c = \sqrt{234} \]
Calculating the square root:
\[ c \approx 15.3 \]
Rounding to the nearest tenth, the length of the hypotenuse \( c \) is approximately 15.3.