To find side $ a $ use Pythagorean Theorem:
$$ s^2 + \frac{ a^2 }{ 4 } = e^2 $$After substituting $s = 60\, \text{cm}$ and $e = 4\, \text{cm}$ we have:
$$ \left( 60\, \text{cm} \right)^{2} + \frac{ a^2 }{ 4 } = \left( 4\, \text{cm} \right)^{2} $$ $$ \frac{ a^2 }{ 4 } = \left( 4\, \text{cm} \right)^{2} - \left( 60\, \text{cm} \right)^{2} $$ $$ \frac{ a^2 }{ 4 } = 16\, \text{cm}^2 - 3600\, \text{cm}^2 $$ $$ a^2 = -3584\, \text{cm}^2 \cdot 4 $$ $$ a^2 = -14336\, \text{cm}^2 $$This equation has no solution $ \Longrightarrow $ The problem has no solution.