Make a the subject get a on its own x(a+2)=7

X (a + 2) = 7

First let's multiply out the brackets on the left side of the equation.
x (a + 2) --> Multiply everything inside the brackets by x.
xa + 2x = 7

Now, both elements of the left-hand side of the equation contain x, so we can divide both sides of the equation by x.

xa + 2x = 7
    a + 2 = 7/x

To find a now, just subtract 2 from both sides.
a = 7/x - 2

X(a + 2) = 7

Divide both sides by x.

x(a + 2) / x = 7 / x

(a + 2) = (7/x)     

a = (7/x)  - 2

Hope this explains it.