Using technology, we determined that Mary’s investment account can be modeled by the function, M(x) = 4.96(2.18)x in thousands of dollars. What was Mary’s initial investment?

well, her initial investment happened at day 0, namely x = 0, let's check,

[tex]\bf M(x)= 4.96(2.18)^x\qquad \boxed{x=0}\qquad M(0)= 4.96(2.18)^0 \\\\\\ M(0)= 4.96(1)\implies M(0)=4.96[/tex]