Any other method than alegebra?

Have other method?

AC Lim summarises it correctly.

You need to realise that the shaded triangles have the same base 10 cm but not their perpendicular heights. But one thing you can see is that the sum of their perpendicular heights is equal to 15 cm.

Sum of areas of triangles
= 1/2 x base x height of bottom triangle + 1/2 x base x height of top triangle
= 1/2 x base x (sum the heights)
= 1/2 x 10 x 15
= 75 cm2

Howsoever the combination of heights for each triangle is picked, the total area remains 75 cm2.

Best way to convince yourself is to try out different values of the heights.

Example ①

If one triangle's height = 10cm, the other's height = 15cm - 10cm = 5cm

Their area = ½ x 10 x 5 + ½ x 10 x 10
= 25 + 50
= 75

(Or ½ x 10 x (5 + 10) = ½ x 10 x 15 = 75)

Example ②

If one triangle's height = 3cm, the other's height = 15cm - 3cm = 12cm

Their area = ½ x 10 x 3 + ½ x 10 x 12
= 15 + 60
= 75

(Or ½ x 10 x (3 + 12) = ½ x 10 x 15 = 75)