To substitute values into an expression on the TI89, use the "" key located in the left column, fourth from the bottom. The first expression at right shows the result of substituting x = 1 into the expression 3x + 2. For a logical expression, such as an equation or inequality, the result is "true" or "false", as shown on the second line. We don't often have to substitute for more than one variable at a time in our course, but to do it you use the "and" operator. 

You can get the "and" operator from the Math (<2nd> <5>) Test menu, as shown, or the Math Base menu, or the Catalog. 
