Explanation:
the area of any triangle is
baseline × height / 2
so, in our case
12 × height / 2 = 48
6 × height = 48
height = 48/6 = 8 units
m with the height and half of the baseline creates a right-angled triangle. and we can use Pythagoras to get m.
c² = a² + b²
c is the Hypotenuse (the side opposite of the 90° angle), which is m for this right-angled triangle.
a and b are the legs, which are in our case the height of the main triangle and half of the baseline of the main triangle.
so we get
m² = 8² + (12/2)² = 8² + 6² = 64 + 36 = 100
m = 10 units