5^(y-1)*5^(x+y)=25 y-x=7 ⇒y=7+x 5^(7+x-1)*5^(x+7+x)=25 5^(6+x)*5^(2x+7)=5² 5^(6+x+2x+7)=5² 5^(3x+13)=5² 3x+13=2 3x=-11 x= -11/3 =-3 2/3 y= 7+(-3 2/3)=3 1/3