For this case we must complete squares:

We add the square of half the coefficient of the term "x",
on both sides of the equation:

According to the perfect square trinomial we have:

Rewriting the expression we have:

ANswer:

Not really sure what you’re saying; but 5x2=10
In a triangle, the three interior angles always add to 180°
m∠R + m∠S + m∠<span>T = 180
</span><span>(2x+10) + (2x+25) + (x-5) = 180
</span>2x+10 + 2x+25 + x-5 = 180
5x + 30 = 180
5x = 150
x = 30