The given expression is
[tex]7=\frac{1}{4}ax[/tex]Solving for a means that we need to isolate that variable.
First, we need to multiply the equation by 4
[tex]7\cdot4=4\cdot\frac{1}{4}ax\rightarrow28=ax[/tex]Second, we divide the equation by x
[tex]\frac{28}{x}=\frac{ax}{x}[/tex]Therefore, the answer is
[tex]a=\frac{28}{x}[/tex]