Solution 2.1:1f

The squaring rule \displaystyle (a+b)^2 = a^2+2ab+b^2 with \displaystyle a=5 and \displaystyle b=4y gives

\displaystyle \qquad \begin{align} (5+4y)^2 &= 5^2+ 2\cdot 5 \cdot 4y +(4y)^2 \\ &= 25+10\cdot 4y + 4^2y^2\\ &= 25+40y+16y^4\\ &= 16y^2 +40y 25 \end{align}