Consider what the area is when a rectangle has a width of 8m and a length of 7m. The area is 8m x 7m, or 56m^2.
Now, multiple the length and width by 5 each, to produce a triangle with the dimensions of 40m and 35m. Multiply the two values together to get 1400m^2.
1400 / 56 = 25, so the area is increased by a factor of 25.