Answer
$2.4\times10^6\;N$.
Work Step by Step
By symmetry, we know that the two forces have the same magnitude F.
Write Newton’s second law for the vertical direction.
$$\Sigma F_y=Fsin\;5^{\circ}+ Fsin\;175^{\circ}-F_{buttress}=0$$
$$F=\frac{F_{buttress}}{ 2\;sin\;5^{\circ}}=\frac{420000\;N}{ 2\;sin\;5^{\circ}}=2.4\times10^6\;N$$