Presumably, in doing (a), (b), and (c), you recognized that y(t) had to be at least 10 when x(t)= 400. That is, you solved the equation x(t)= 400 for t, put that into y(t) and showed that y was larger than 10. Do the same, leaving theta as a variable: t= 400/(v0cos(theta), so y= 3+ v0 sin(theta)(400/v0 cos(theta))- 16(400/v0cos(theta))^2= 10. Presumably you have already found v0 in feet per second so that is an equation for theta. Solve that equation.