To apply the Pythagorean Theorem, we can use the formula:
a^2 + b^2 = c^2
where a and b are the lengths of the two legs of the triangle, and c is the length of the hypotenuse.
In this case, the base (a) has a value of 3, the hypotenuse (c) has a value of 15, and we are trying to find the length of the perpendicular side (b).
We can set up the equation as follows:
3^2 + b^2 = 15^2
9 + b^2 = 225
b^2 = 216
Taking the square root of both sides to find b:
b ≈ 14.7
Therefore, the length of the unknown side is approximately 14.7.