How do u get the answers for 18 and 19

18) 90-81=9

    Complementary angles are angles that add up to 90 degrees. If the angle is 81 degrees, then its complementary angle will be 9 degrees

19) 180-20=160

    Supplementary angles are angles that add up to 180 degrees. If the angle is 20 degrees, then its supplementary angle will be 160 degrees.

i think it's ф or x=6,-6

step-by-step explanation:

-x^2=-36

x^2=-36

ф

or

if you mean by : -(x^2)=-36

it'll be: x^2=36

x=6,x=-6

hope it'll ! have a good day!

you for asking!