Predict the result
Evaluate x plus seven when x is three and when x is twelve.
Can the same expression have different values?

Set up the model
Substitute the stated value for the variable.
Then follow the indicated arithmetic.

Work the example
When x is three, calculate three plus seven.
That gives ten.
When x is twelve, twelve plus seven gives nineteen.
Changing the input changes the expression’s value.

Check the result
Now evaluate y plus four when y is six and when y is fifteen.
Take eight seconds to think before the answer appears.
[Eight-second silent pause to work; the answer remains hidden.]
Six plus four gives ten.
Fifteen plus four gives nineteen.

Interpret the result
Evaluation means substitution followed by calculation.
Use the original task and check that your result fits its meaning.