@hollyh10 .... In D11, you can enter either of the following formulas:
=TREND($C$4:$C$8, $F$4:$F$8, D10)
=FORECAST(D10, $C$4:$C$8, $F$4:$F$8)
In this case, the TREND formula does __not__ need to be array-entered, since that use of TREND returns a single value (since D10 is a single cell), not an array of values (as it would for a range like D10:D20).
I prefer the FORECAST formula, because you do not require all the bells and whistles that TREND can provide. KISS.
For merged cells, refer to the upper-left cell. Do not refer to the range of cells that comprise "a" merged cell.