How do you do this question?

S(50) = 0

Step-by-step explanation:

The initial condition is S(0) = 0, so start at the origin.  The slope there is approximately 1.  So as t increases by a small amount, S increases by the same amount.  Continuing this logic, we end up tracing the vector field until we get to t=50, where S is approximately 0.

Each little tickmark is a slope value telling you which direction to go. If you place yourself at the origin (0,0) then the tick mark here tells us to go roughly in the northeast direction. As you step forward to the northeast just a tiny bit, the slope value changes to aim more toward the east (rather than north). For more information, check out Euler's method. Specifically it's Euler's method dealing with slope fields.

The diagram below shows what could be each step of this process. Each red dot is a stepping stone, so to speak, going from (0,0) to (50,0). This is an approximation. Without the actual function F(t, S), it's hard to say where exactly S(50) is located. But S(50) = 0 is a fairly good guess I think. MathPhys has a great diagram showing what could be the solution for S(t) given the initial condition S(0) = 0. Note how they drew a curve through all of the red points I've marked.

check above link.

graphically?   well, you graph them both, the solutions is where they both intersect each other, since there are two solutions, meaning they will meet twice, or namely the linear will run into the quadratic curve twice, and where they meet, those x,y coordinates are the solutions to the system.