The diagram representing the forces is shown below
The triangle formed is a right angle triangle. x represents maginitude of the third force. To find x, we would apply the pythagorean theorem which is expressed as
hypotenuse^2 = one leg^2 + other leg^2
From the triangle,
one leg = 75
other leg = 60
hypotenuse = x
By applying the theorem,
x^2 = 60^2 + 75^2 = 3600 + 5625 = 9225
x = square root of 9225
x = 96
The magnitude of the third force is 96 N