The triangle we start with is famously right-angled and so has an area of 3x4/2 = 6.
Rotate the triangle so that the hypotenuse is now the base and the right angle is the apex. Since the area is still 6, the height in this orientation must be such that 5xh/2=6, so the height is 2.4 units.
Bring that apex directly downwards towards the base of length 5. It will now be 0.4 units above the base.
The new area is therefore 5x0.4/2, which is equal to 1.
