If you apply a dilation by a scale factor of 2.5 with a center of dilation at vertex b, the point b' will be the same point b (it is the center of the dilation so it does not change).
Then:
b' = b
The length of segment a'b' = 2.5 * the length of segment ab
The length of segment c'b' = 2.5 * the length of segment cb
Given the similarity properties, the height of triangle a'b'c' is 2.5 times the height of the triangle abc.
You can take ab as the base of the original triangle and a'b' as the base of the new triangle.
In that case, the area of the original triangle is: ab*height
And the area of the new triangle is (2.5ab) * (2.5 height) = (2.5)^2 * ab * height = 6.25*(ab * height)
=> new area = 6.25 * original area.
Answer: area of a'b'c' is 6.25 times the area of abc.