exam Maths Puzzle Numerical Ability

Find the pH if H+ = 2.68 * 10^-3 mole per liter. (pH = - logH+ or 10^-pH = H+)

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exam Other Question

I'm not sure how to solve this problem. The sum of two numbers is 25. The sum of their squares are 313. Find the numbers.

My work:
x+y=25
x^2+y^2=313

x=x
y=25-x

x^2+(25-x)^2=313
x^2+625+x^2-50x=313
2x^2-50x+312=0
2(x^2-25x+156)=0