Hi there!
Work out the parenthesis.
Combine like terms.
Subtract 9 from both sides.
Subtract 4x from both sides
Divide both sides by -9
#21 is 95 degrees
#22 is B
The answer is the option A, which is: A. Equilateral triangles.
The explanation for this answer is shown below:
By definition, a tetrahedron is a solid in three dimensions that has: six sides, four vertices and four triangular faces. These faces are equilateral triangles, which means that all the sides are equal and all the interior angles measure 
degrees.
The tetrahedron is also called "triangular pyramid".
