VBA + LINEST Function

Assuming I understand what you are trying to do, the reason you are having trouble is because VBA cannot understand Excel's array constant syntax (the ^{1,2} part of the desired LINEST() formula). I see two basic approaches to a solution to this kind of problem:

1) Since VBA cannot handle that syntax, shift the regression so it is performed in the Excel rather than VBA. In the context of your code, this might be accomplished by assigning the LINEST() formula to a string variable, then assigning that variable to the formulaarray property of the output range. This would be the same as entering the formula in the spreadsheet, so that the regression is performed in the spreadsheet.

2) With a fuller understanding of what the range^{1,2} syntax does in Excel, find some way to recreate the effect in VBA. In Excel, range^{1,2} creates a two dimensional matrix where the first column contains range^1 and the second column contains range^2.
2a) One way would be to use spreadsheet formulas and create this 2D matrix in the spreadsheet. The formulas should be simple (=J3,=J3^2) copied down. The use this range for the src1 object that is being used by the LINEST() function.
2b) Another possibility, use a loop in VBA to create a 2D array that will contain these values. Then assign this array to src1 (src1 will need to be dimensioned as a variant or will need to be dimensioned as a 2D array).

Which approach you choose will depend on other constraints and considerations.