To simplify the expression \((8m)(7m)\), you can follow these steps:
- Multiply the coefficients (the numerical parts): \(8 \times 7 = 56\).
- Multiply the variable parts: \(m \times m = m^2\).
Putting it all together, you get:
\[ (8m)(7m) = 56m^2 \]
So, the simplified expression is:
\[ \boxed{56m^2} \]