We can say that the triangles is similar if the slope of the hypotenuse is the same.
The slope is Vertical sides divided by the horizontal side.
The upper triangle has vertical side of 3 and a horizontal side of 2 (By counting)
The slope will be m = 3/2
The lower triangle has a vertical side of 6 and a horizontal side of 4.
The slope will be m = 6/4 = 3/2
Since the slope is the same, the triangles are similar. and the slope is m = 3/2