1. Let A, B and C denote the vertices of the triangle you are looking for.

2. Draw the triangle LMN.

3. The side of triangle ABC which passes through L must be parallel to MN:

Calculating the slope MN:

Calculating the equation of the line through L:

4. Do just the same with the points M and N. You then have 3 equations of three lines.

5. Calculate the coordinates of the intersection points of these three lines.