To find an equivalent algebraic expression to \(2.1x + 3.5y\) from the given options, we can first simplify \(2.1x + 3.5y\).
We can factor out \(0.7\) from \(2.1\) and \(3.5\):
\[ 2.1 = 0.7 \times 3 \] \[ 3.5 = 0.7 \times 5 \]
The expression \(2.1x + 3.5y\) can then be rewritten as:
\[ 2.1x + 3.5y = 0.7(3x + 5y) \]
Now let's check this against the options provided:
- \(0.7(3x - 5y)\)
- \(0.7(3x + 5y)\)
- \(0.7(5x - 3y)\)
- \(0.7(5x + 3y)\)
The expression \(0.7(3x + 5y)\) matches our derived expression. Therefore, the equivalent algebraic expression is:
\[ \boxed{0.7(3x + 5y)} \]