You have three lines: y = 2.41x + 120.45, y = 1.94x + 134.26, and y = mx + 25.45
See where the three lines intersect, and that will give you three points. Find the distance between each point. Then use Heron's formula to calculate area.
Distance between two points :
Heron's formula for area of a triangle: where , and the three sides of the triangle have lengths .