If a spring has a constant of 4 (kg)/s^2, how much work will it take to extend the spring by 78 cm ?

Work done in extending a spring ,is stored in it as elastic potential energy,and that is expressed as $\frac{1}{2} k {x}^{2}$(where, $k$ is the spring constant and $x$ is the amount by which it is extended)
Given, $k = 4 , x = \frac{78}{100}$
So, work done = $\frac{1}{2} \cdot 4 {\left(\frac{78}{100}\right)}^{2} = 1.217 J$