#### Answer

$4.5$

#### Work Step by Step

$a_{m}-a_{n}=\left( m-n\right) d\Rightarrow a_{1a}-a_{10}=\left( 14-10\right) d\Rightarrow -73.5-\left( -49.5\right) =4d\Rightarrow d=-6;a_{10}=a_{1}+9d\Rightarrow -49.5=a_{1}+\left( -54\right) \Rightarrow a_{1}=4.5$