To find the angle between two forces, we can use the cosine formula:

cos(theta) = (a^2 + b^2 - c^2) / (2ab)

where a and b are the magnitudes of the forces, and c is the magnitude of the resultant force.

In this case, a = 3N, b = 4N, and c = 5N.

cos(theta) = (3^2 + 4^2 - 5^2) / (2 * 3 * 4) = (9 + 16 - 25) / 24 = 0

Since the cosine of the angle is 0, the angle must be 90 degrees.