Because we know that the slope of the straight line passing through the height is -3, we can know from the vertical relation k 1*k2+ 1=0 that the slope of the straight line passing through (2, -7) is 1/3, and the equation is: x-3y-23 = 0 ①;

① The intersection of simultaneous midline equation x+2y+7=0 is the other vertex (5,-6) of the triangle;

Let the other vertex be A(a, b), then the midpoint B((a+2)/2, (b -7)/2) between a and (2,-7) passes through the midline equation X+2Y+7 = 0 ②;

Bring point B into equation ② and get the equation: A+2B+2 = 0 ③;

The equation of the straight line that brings A to the height is 3a+b+11= 0 ④;

The solutions of ③ and ④ are a =-4 and b =1; Namely A(-4,1);

Then the equations of the other two straight lines are obtained by two-point formula: 7x-9y+37=0, 4x+3y-19 = 0;

So the straight lines of the three sides of the triangle are: x-3y-23=0, 7x-9y+37=0, 4x+3y- 19=0.

Supplement: the writing is not very standardized, because it is hard to say without drawing. It is easier to understand by drawing a triangle when reading by yourself, and then standardize and specialize the language by yourself.