How do you simplify by multiplying (x+10)^2?

it's easy. As suggested, I first want to show where ${\left(a + b\right)}^{2}$ is equal to.
${\left(a + b\right)}^{2} = \left(a + b\right) \cdot \left(a + b\right) = a \cdot \left(a + b\right) + b \cdot \left(a + b\right) = a \cdot a + b \cdot a + a \cdot b + b \cdot b = {a}^{2} + 2 a b + {b}^{2}$
${\left(x + 10\right)}^{2} = {x}^{2} + 2 \cdot x \cdot 10 + {10}^{2} = {x}^{2} + 20 x + 100$