There are a couple of ways to approach this, Turgul's post suggest the following:
Let $B' = \{u_1,u_2,b_3,b_4,b_5\}$.
Since $b_1 = u_1 - u_2 - 2b_3 - 2b_5$ and:
$b_2 = u_2 + b_3 - b_4 + b_5$.
It should be clear this set spans $V$, and since it has 5 elements must therefore be a basis for $V$ (this saves us the rigamarole of showing linear independence).
Applying the quotient map to this set gives us the set:
$\{b_3 + U, b_4 + U, b_5 + U\}$ and it suffices to show this is a basis for $V/U$ (the first two elements map to $u_1 + U = u_2 + U = 0 + U = U$, the 0-vector of $V/U$).
Since $\text{dim}(V/U) = \text{dim}(V) - \text{dim}(U) = 5 - 2 = 3$ it suffices to show that this set is LI.
So suppose that (for $c_i \in F$) we have:
$c_1(b_3 + U) + c_2(b_4 + U) + c_3(b_5 + U) = U$
This is equivalent to saying:
$c_1b_3 + c_2b_4 + c_3b_5 \in U$, that is:
$c_1b_3 + c_2b_4 + c_3b_5 = a_1(b_1 + b_2 + b_3 + b_4 + b_5) + a_2(b_2 - b_3 + b_4 - b_5)$, or equivalently:
$-a_1b_1 - (a_1 + a_2)b_2 + (c_1 - a_1 + a_2)b_3 + (c_2 - a_1 - a_2)b_4 + (c_3 - a_1 + a_2)b_5 = 0$.
By the LI of $B$, we have:
$a_1 = 0$
$a_1 + a_2 = 0$
$c_1 - a_1 + a_2 = 0$
$c_2 - a_1 - a_2 = 0$
$c_3 - a_1 + a_2 = 0$
From the first and second equation, we see that $a_1 = a_2 = 0$, which then shows that all the $c_i = 0$.
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One could just form the set $\{b_1 + U, b_2 + U,b _3 + U, b_4 + U, b_5 + U\}$ which clearly spans $V/U$, and try to form a basis out of it, somehow.
Since $b_1 + b_2 + b_3 + b_4 + b_5 \in U$, this means:
$b_1 + U = -b_2 + U - (b_3 + U) - (b_4 + U) - (b_5 + U)$ so we can safely remove $b_1 + U$ from the set above without changing the span.
Similarly, since:
$b_2 - b_3 + b_4 - b_5 \in U$, we have:
$b_2 + U = b_3 + U - (b_4 + U) + b_5 + U$, so we may also likewise remove $b_2 + U$ without affecting the span.
Since $\{b_3 + U,b_4 + U,b_5 + U\}$ spans $V/U$ and we have shown LI above, we therefore have a basis.