Remember from class we discussed the difference between numerical expressions and algebraic expressions. Algebraic expressions contain a variable. Also remember that expressions are part of equations. An equation is demonstrating two expressions that are equal. For example the equation y + 6 = x + 5 has two algebraic expressions which you know are equivalent given the equal sign.
To evaluate algebraic expressions you must know the value of the variable. The values of the variables will always be given. Here's an example:
Evaluate the expression 4b + 3c - 10 for b=5 and c=2.
To evaluate simply substitute the value of the variable and follow order of operations. In class, I expect to see the substitution. This is the expectation:
4(5) + 3(2) - 10 This shows the substitution
20 + 6 - 10 This demonstrates following the orders of operations
26 - 10
Evaluate x^y - xyz + 6(z - y) when x=2, y=3, and z=(-1)
2^3 - (2)(3)(-1) + 6(-1 - 3)
8 - (-6) + 6(-4)
8 + 6 + (-24)
14 + (-24)