Solve the equation using the quadratic formula : x^2-10x+25=18

X^2 - 10x + 25 = 18. Send 18 to the other side. The equation becomes : 
x^2 -10x + 7 = 0 Now solve for x, using the quadratic formula.
x = (-b +- /b^2 - 4ac)/2a      
{+- (plus minus)  /(square root) } Note: the square root is 4 the whole (b^2 - 4ac) from equation :  x^2 -10x + 7 = 0    a=1, b= -10, c=7   Replace the values of a,b&c in the quadratic equation. 
x = (10+-/((-10)^2 - (4*1*7))/2) = ( 10 + /72 )/2. [Simplify it on your own, here it seems to be pretty weird with these: +- and / as square root.] Well, at the end you should obtain x=(10 +- square root of 72) divided by 2
either: x = (10 + /72) divided by 2 = 9.24
or      : x = ( 10 - /72) divided by 2 = 0.757 
Pretty long explanation