Question 1074679
It helps to draw this.
The person is looking at each peak on a leg of a right triangle.  The hypotenuse is the distance between the mountains.  
x^2+(x+5)^2=25^2
x^2+x^2+10x+25=625
2x^2+10x-600=0
x^2+5x-300=0
(x+20)(x-15)=0
only positive root is x=15
x+5=20
The vista point is 15 miles from one mountain peak and 20 miles from the other.