How many different 99-letter permutations can be formed from 77 identical h's and two identical t's?

for this case we have by definition, if two lines are perpendicular, then the product of their slopes is -1.

then, given the following line:

so,

we are looking for m_ {2}:

then, the line is given by:

we find "b" replacing the given point:

finally, the equation is:

option c

thats not a system of equations, that is a solution