Getting a balanced exposure when the sky is much brighter than the ground requires the use of a split neutral density filter, also called ND graduated filters or ND grads. The filter is grey on one half and clear on the other. The neutral grey tone helps darken part of the scene to balance the exposure. As you may know, reflections appear a bit darker than the scene reflected, so the filter should never be dark enough to make the scene appear the same brightness or darker than the reflection. In this case I goofed a bit. The mountain is slightly darker than the reflected mountain. Also, the scan didn't capture the colours as well as on the original slide. Without Photoshop I can't really do enough to correct the colours.