$M_O = y_AF_x - x_AF_y$

$400 = y_A(361)(\frac{3}{\sqrt{13}}) - 2(361)(\frac{2}{\sqrt{13}})$

$y_A = 2.665 \, \text{ ft}$ *answer*

Y-intercept of the line of action of force F

$M_O = F_xb$

$400 = 361(\frac{3}{\sqrt{13}})b$

$b = 1.332 \, \text{ ft above point O}$ *answer*

X-intercept of the line of action of force F

$M_O = F_ya$

$400 = 361(\frac{2}{\sqrt{13}})a$

$a = 1.998 \, \text{ ft to the left of point O}$ *answer*

How did you get 2/Sqrt13 as the dimension in y direction?

You have to break down the Force F into its components: Fx and Fy. Arrow F has a 2:3 slope ( count the number of squares). Its hypotenuse by P-theorem is the square root of 13. To resolve Fy you have to multiply F with 2/square root of 13. Hope this helps.

The slope appears to be 2:3, but that information is not provided in the problem statement or the diagram, so the solution uses an unjustified assumption.