Explanation:
x = the area in black
each triangle have area = 80
the area of the 3 triangles without combining them = 80 × 3 = 240
the total area = 210
the total area in the figure = 3 triangles area - 2x
because we have calculated 1 black area not the three of them so we need to add its area once and get rid of the others by subtracting them from the sum of the 3 triangles area
240 - 2x = 210
2x = 30
x = 15
hope that helps