Can you point out any similarity or difference between the expressions in Example 1 (Refer to NCERT page no. 16) and Example 3 (Refer to NCERT page no. 17)?                                                                                                             [Page No. 17]

Similarities:

Both are algebraic expressions made of terms with numerical coefficients.

Both contain a plain $x$ term ($4x$ in Example 1, $10x$ in Example 3).

Differences:

Number of variables: Example 1 uses two variables ($x$ and $y$); Example 3 uses only one ($x$).

Highest power: Example 1 has only first powers, so it is linear;
Example 3 contains $x^2$, a second power, so it is quadratic $\rightarrow$ not linear.

Example 1 has a constant term (3); Example 3 has none.