To solve this equation, we will combine like terms and isolate the variable x.

3.5x + 8.4 = 7x - 2.1x + 3
3.5x + 8.4 = 4.9x + 3

Next, we will move all the terms with x to one side of the equation:

3.5x - 4.9x = 3 - 8.4
-1.4x = -5.4

Now, we will solve for x by dividing both sides by -1.4:

x = -5.4 / -1.4
x ≈ 3.857

Therefore, the solution to the equation 3.5x + 8.4 = 7x - 2.1x + 3 is x ≈ 3.857.