Chapter 1 - Some Basic Concepts of Arithmetic and Algebra - 1.1 - Numerical and Algebraic Expressions - Problem Set 1.1 - Page 7: 58

$A=18$

We start with the equation for the area of a triangle: $A=\frac{bh}{2}$ We plug in the values for base and height: $A=\frac{9\times4}{2}$ To simplify, we follow the order of operations. First, we multiply above the fraction bar: $A=\frac{36}{2}$ Then, we divide: $A=18$