Only 20%of earths water is useable
It would be t=0.8, mainly bc you subtract two from six , that reduces it to four
then you divide four by five and it should give you your answer
and to chack your work put .8 where t was and solve
X^4 - 5x^2 - 36 = 0
(x^2 - 9)(x^2 + 4) = 0
(x-3)(x+3)(x-2)(x+2) = 0
x = -3, -2, 2, 3