Answer
$309^{\circ}$
Work Step by Step
The angles which has the same initial side and terminal sides are known as the co-terminal angles. In order to find the co-terminal angles we will have to add or subtract $360^{\circ}$ or $2π$ to each angle (but selecting on whether the given angle is in degrees or radians)
In order to compute co-terminal angle we will add $360^{\circ}$ to the given angle $-51^{\circ}$.
Therefore, we have $-51^{\circ}+360^{\circ}=309^{\circ}$
