We know the line runs through -2,3 and 2,5.
it also runs through 2, 5 and 6,k.
since all points are on the line, they're colinear, and therefore the line runs also through -2,3 and 6,k.
keeping in mind that line maintains a constant slope, therefore, the slope for -2,3 and 2,5, has to be the same slope as for 2,5 and 6,k.
what is the slope of -2,3 and 2,5 anyway?

and since we know the slope of 2,5 and 6,k is the same, then,