To solve for a, you must get a by itself on one side of the equation. Now, a is "joined" to m by multiplication. To "separate" them, use the inverse operation, division.
Divide both sides of the equation by m.
[tex]F=ma \\ \frac{F}{m}=\frac{ma}{m} \\ \frac{F}{m}=a \\ a=\frac{F}{m}[/tex]
