You've expanded $\displaystyle -2(5-\frac{t}{4})^2$ incorrectly.

You're going to need to get this algebra thing licked, mate

$\displaystyle x(a+b)^2=x(a+b)(a+b)$

$\displaystyle =x(a^2+ab+ab+b^2)$

$\displaystyle =x(a^2+2ab+b^2)$

$\displaystyle =xa^2+2xab+xb^2$

Notice that the equation you're interested is in the form $\displaystyle x(a-b)^2=x(a-b)(a-b)$, so you should get $\displaystyle xa^2-2xab+xb^2$ as a solution.