Expand and write in descending powers of $a$. For example: $\boxed{\phantom{A}}\;a^{\color{red}{4}}+\boxed{\phantom{A}}\; a^{\color{red}{3}}b+\boxed{\phantom{A}}\;a^{\color{red}{2}}b^2+\boxed{\phantom{A}}\;ab^3+\boxed{\phantom{A}}\;b^4$
$\left(a+b\right)^1=$
$\left(a+b\right)^2=$
$\left(a+b\right)^3=$
$\left(a+b\right)^4=$
$a+b$
Do you notice a pattern with the coefficients and the exponents?