Explanation:
the area of a triangle is
base × height / 2
in our case
base × height / 2 = 196 mm²
base × height = 392
base = height / 2
so, using that in our main equation :
(height / 2) × height = 392
height² = 784
height = sqrt(784) = 28 mm
base = height / 2 = 28/2 = 14 mm