Correct. Regroup the terms using the commutative and associative properties: $\displaystyle{\left({3}{x}+{5}{x}\right)}+{\left({4}+{1}\right)}\usepackage{cmbright}\Huge\$

Combine like terms: $\displaystyle{8}{x}+{5}\usepackage{cmbright}\Huge\$

The missing coefficient is 8.