X2 - 8x + 15 = 0
quadratic formula

The quadratic formula is x=(-b+-sqrt(b^2-4(a)(c)))/2a

Using the quadratic formula, we get:

x=(-(-8)+-sqrt((-8)^2-4(15)(1)))/2(1)

x=(8+-sqrt(64-60))/2

x=(8+-sqrt(4))/2

x=(8+-2)/2

x=10/2 and x=6/2

x=5 and x=3

We could do it way faster by completing the square:

x^2-8x+15=0

(x-5)(x-3)=0

x=5 and x=3

Hope both methods make sense! BTW, the +- thing means add or subtract