If p = (3a + 5)/(4 - a), make a the subject of the formula

You want to end up an equation that show what a is in terms of p. To make things simpler, we want to get rid of the fraction on the right by multiplying both sides by (4 - a). If we do this we end up with:
p(4 - a) = 3a + 5
Simplifying the right, you get:
4p - pa = 3a + 5
You want all the a's on one side and the p's on the other, so you can add pa to both sides or subtract 3a , Let's add pa to both sides as positive numbers are nicer to deal with. Then you want everything else that isn't an a on the other side, so take away 5. No we have: 3a + pa = 4p - 5
Next you want to factorise the expression on the right by taking out a factor of a
a(3 + p) = 4p - 5
Divide both sides by 3 + p to find an expression for a
a = (4p - 5)/(3 + p), final answer