Which number in written in scientific notation? (big points reward)

a: 7.8 x 10^-5
b: 3.2 x 100^2
c: -5 x 10^-12
d: 0.84 x 10^6

The decibel level of sound is 50 db greater on a busy street than in a quiet room where the intensity of sound is watt/m2. the level of sound in the quiet room is db, and the intensity of sound in the busy street is watt/m2. use the formula , where is the sound level in decibels, i is the intensity of sound, and is the smallest sound intensity that can be heard by the human ear (roughly equal to watts/m2).

How to choose the sign of the radical in the denominator of the formula for the distance from a point to a line.

Jack did 3/5 of all problems on his weekend homework before sunday. on sunday he solved 1/3 of what was left and the last 4 problems. how many problems were assigned for the weekend?