we have the equation
2x-3y=4
Remember that
if the ordered pair is a solution of the given equation, then the ordered pair must satisfy the given equation
we have the ordered pair (-1,a)
substitute the given coordinates in the equation
2(-1)-3(a)=4
-2-3a=4
solve for a
3a=-2-4
3a=-6
a=-2
therefore
the missing coordinate is -2