Answer
$\Phi = 85.2~Wb$
Work Step by Step
We can find the height of the bar above the vertex:
$h = (5.20~m/s)(3.00~s) = 15.6~m$
We can find the area of the right angle triangle form by the bar and rails:
$A = \frac{1}{2}bh$
$A = \frac{1}{2}(2h)(h)$
$A = h^2$
$A = (15.6~m)^2$
$A = 243.4~m^2$
We can find the magnetic flux through the triangle:
$\Phi = A~B$
$\Phi = (243.4~m^2)(0.350~T)$
$\Phi = 85.2~Wb$
