1.1. The function $\text{f}$ is defined by

$\text{f}(x)={x}^{3}+{x}^{2}-5xx\inℝ$

Find $\text{f}'(x)$.

1.2. Using algebra, solve the equation $3{x}^{2}+2x-5=0$.

1.3. Hence find the set of values of $x$ for which $\text{f}$ is a decreasing function.