1. The problem asks to calculate the value of the expression $x - 3$ for two values of $x$: a) $x=2$ and b) $x=7$. 2. The formula used is simply substitution: replace $x$ with the given value and perform the arithmetic. 3. For a) when $x=2$: $$x - 3 = 2 - 3 = -1$$ 4. For b) when $x=7$: $$x - 3 = 7 - 3 = 4$$ 5. Explanation: To find the value of an expression for a given variable, substitute the variable with the number and then calculate the result. Final answers: a) $-1$ b) $4$