how do I solve for a for the indicated variable

Answer:
16)
[tex]a=-\dfrac{3}{4u}[/tex] .......as required
18)
[tex]a=\dfrac{u}{4}[/tex]..........as required
Step-by-step explanation:
16)
[tex]u=-\dfrac{3}{4a} ,for\ a[/tex]
Multiply both the side by 'a' and divide both the side by 'u'
[tex]\dfrac{a}{u}\times u=-\dfrac{3}{4a}\times \dfrac{a}{u}[/tex]
Cancel 'u'on Left side and 'a' on right side we get
[tex]a=-\dfrac{3}{4u}[/tex] .......as required
18)
[tex]u=4a[/tex].......Given
Dividing by 4 on both the side we get
[tex]\dfrac{u}{4} =\dfrac{4a}{4} \\[/tex]
Cancel 4 on right side we get
[tex]a=\dfrac{u}{4}[/tex]..........as required