To find side $ b $ use Pythagorean Theorem:
$$ a^2 + b^2 = d^2 $$After substituting $a = 10\, \text{cm}$ and $d = 26\, \text{cm}$ we have:
$$ \left( 10\, \text{cm} \right)^{2} + b^2 = \left( 26\, \text{cm} \right)^{2} $$ $$ b^2 = \left( 26\, \text{cm} \right)^{2} - \left( 10\, \text{cm} \right)^{2} $$ $$ b^2 = 676\, \text{cm}^2 - 100\, \text{cm}^2 $$ $$ b^2 = 576\, \text{cm}^2 $$ $$ b = \sqrt{ 576\, \text{cm}^2 } $$$$ b = 24\, \text{cm} $$