"Evaluate by substitution" sounds fancy, but it's two moves: swap the letter for its value, then work out the sum using the order of operations. That's the lot.
The Two Moves
One — substitute. Wherever the letter appears, write the number in its place. Keep the hidden times sign: 3a becomes 3 × (the value), not the two numbers shoved together. Brackets around the value are a tidy habit, especially once negatives turn up.
Two — evaluate. Now it's just arithmetic. Run the order of operations — brackets, then indices, then × and ÷, then + and −.
A Worked One
Evaluate 3a − 4 when a = 6. Substitute: 3 × 6 − 4. Evaluate: do the multiply first, 18 − 4 = 14. So the answer is 14. Notice the order mattered — multiply before the subtraction.
Brackets Do Their Job
For 2(x + 5) with x = 3: substitute to get 2(3 + 5), do the bracket first, 2 × 8 = 16. The bracket tells you what to add before you multiply — the order of operations is quietly steering the whole thing.