9. If \( x-y=4 \) and \( x y=21 \), then \( x^{3}-y^{3} \) is \( \begin{array}{ll}\text { (a) } 361 & \text { (b) } 316 \\ \text { (c) }-188 & \text { (d) } 360\end{array} \)

What is the value of one third the determinant of \( \left[\begin{array}{cccc}2 & 3 & 0 & 11 \\ 3 & 5 & -1 & 10 \\ 0 & -6 & 2 & 4 \\ 2 & 4 & 7 & -5\end{array}\right] \) ?

The cost price of a pen is Rs \( (3 x+5) \) and is selling price is \( \operatorname{Rs}(4 x+3) \) Write the formulato find protit.

What should be, subtracted from \( 8 x-3 y+4 \) to get \( 3 x-2 y+4 \).

\( 1 \leftarrow \quad \begin{array}{l}\text { Solve the system of equations by the substitution method. } \\ \begin{aligned} 4 x+2 y & =-154 \\ x-5 y & =0\end{aligned}\end{array} \)

\( 8 x y \) is a monomial expression. Is the statement true or fause?