Supposing you want this equation to be solved for a: we start with
[tex]-\dfrac{a}{5}+21=23[/tex]
Subtract 21 from both sides:
[tex]-\dfrac{a}{5}=2[/tex]
Multiply both sides by 5:
[tex]-a=10[/tex]
Change sign to both sides:
[tex]a=-10[/tex]