Triangle-Angle-Bisector Theorem : An angle bisector is a straight line drawn from the vertex of a triangle to its opposite side in such a way, that it divides the angle into two equal or congruent angles in such a way that the ratio of the two line-segments is proportional to the ratio of the other two sides.
In the given triangle FIH, line IG is divide the angle I
So, from the defination of angle bisector theorem : The ratio of two line segments is proportional to the ratio of the other two sides.i.e.

In the given figure : IF = x,