Assuming that a, b and c are all vectors:
The dot product of two vectors [tex]x\cdot y[/tex] returns a number.
Let [tex]b\cdot c = k[/tex], for some [tex]k \in \mathbb{R}[/tex]
The expression becomes
[tex]a\cdot (b\cdot c) = a\cdot k[/tex]
This is the multiplication between a vector and a scalar, and it returns a multiple of the vector.
