if my example is a y=kx one and y=-5 and x=10 which are directly proportional why can't I just isolate the "k" (constant) and solve as a regular algebraic equation?
yes, you can
-5 = k(10)
k = -5/10 = -1/2
it makes no difference when you isolate and solve for k ...
y = kx
k = y/x = -5/10 = -1/2