Step-by-step explanation:
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Y varies directly with x m eans
y=kx where k is constant
y=40
x=16
find k
40=k16
divide both sides by 16
5/2=k
y=5/2x
y=-5 find x
-5=5/2x
times 2/5 both sides to cancle fraciton (5/2 times 2/5=10/10=1)
-10/5=x
-2=x
x=-2
Example...
4^x = 16
log(4^x) = log(16)
x*log(4) = log(16)
x = log(16) / log(4)