To solve this expression, we need to apply the order of operations (PEMDAS) which stands for Parentheses, Exponents, Multiplication and Division (from left to right), and Addition and Subtraction (from left to right).

Given expression: 6x - 7x(3.5 - (-1)) + 4

First, simplify the expression inside the parentheses:
3.5 - (-1) = 3.5 + 1 = 4.5

Now the expression becomes:
6x - 7x(4.5) + 4

Next, perform the multiplication:
6x - 31.5x + 4

Combine like terms:
-25.5x + 4

So, the simplified form of the expression 6x - 7x(3.5 - (-1)) + 4 is -25.5x + 4.