What does this triangle pointing right symbol mean?

Question

In the third step below, what does the little triangle pointing right mean?

Sum of roots: \begin{align}\alpha+\beta&=-\frac 53 \cdots \tag{1}\\[6pt] \alpha-\beta&=\frac 73\cdots \tag{2}\end{align}

$(1)+(2)$: \begin{align}2\alpha&=\frac 23\\[6pt] \alpha&=\frac 13\cdots\tag{3}\end{align}

$(3) \triangleright(1)$:

\begin{align}\left(\frac 13\right)^2+5\left(\frac 13\right)+k&=0\\ k&=-2\end{align}