Working out the expression 7(3ˣ) is first to calculate the value of 3ˣ then multiply the answer to 7.
For example, let x = 2
Then 7(3ˣ) = 7(3²) = 7 × 9 = 63
With the same value of x = 2, calculating 21ˣ = 21² = 441, which isn't the same answer with the expression 7(3ˣ)