how can I do this ???

Answer:
The triangle EFG can be constructed with the following specifications: [tex]EF = 8\,cm[/tex], [tex]FG = 7\,cm[/tex], [tex]EG = 12\,cm[/tex], [tex]E \approx 34.093^{\circ}[/tex], [tex]F \approx 106.068^{\circ}[/tex], [tex]G \approx 39.838^{\circ}[/tex].
Step-by-step explanation:
A triangle is formed by either knowing the lengths of its three sides or knowing two angles and the length of a side or knowing a angle and the lengths of two sides. By Geometry, we know that sum of internal angles in triangles equals 180°. In order to construct this triangle, we need to know the measures of angles E, F and G by means of the Law of Cosine:
Angle E
[tex]E = \cos^{-1}\left[\frac{(7\,cm)^{2}-(12\,cm)^{2}-(8\,cm)^{2}}{-2\cdot (12\,cm)\cdot (8\,cm)} \right][/tex]
[tex]E \approx 34.093^{\circ}[/tex]
Angle F
[tex]F = \cos^{-1}\left[\frac{(12\,cm)^{2} - (8\,cm)^{2} - (7\,cm)^{2}}{-2\cdot (8\,cm)\cdot (7\,cm)} \right][/tex]
[tex]F \approx 106.068^{\circ}[/tex]
Angle G
[tex]G = \cos^{-1}\left[\frac{(8\,cm)^{2}-(12\,cm)^{2}-(7\,cm)^{2}}{-2\cdot (12\,cm)\cdot (7\,cm)} \right][/tex]
[tex]G \approx 39.838^{\circ}[/tex]
The triangle EFG can be constructed with the following specifications: [tex]EF = 8\,cm[/tex], [tex]FG = 7\,cm[/tex], [tex]EG = 12\,cm[/tex], [tex]E \approx 34.093^{\circ}[/tex], [tex]F \approx 106.068^{\circ}[/tex], [tex]G \approx 39.838^{\circ}[/tex].