i am going to say it in different order
dcshioaeron
The answer is a septahedron, 11 letter answer
triangular prism
vv
An 11 sided shape is an Eptogon
An 11-sided 3D shape is a hendecahedron.
Tetrahedron
semicircles
a bottomhead
The answer is a septahedron, 11 letter answer
a symmetrical shape
triangular prism
a bottomhead
Ss
The shape you're looking for is "seesaw." It has 11 letters, possesses at least one line of symmetry, and starts with the letter "s." A seesaw can be symmetrical along its central axis.
I cannot think of any 2-d shape. The 3-d shape with four faces is a tetrahedron.
Septahedron
a isosceles triangle has at least 1 line of symmetry but only has 9 letters