One way to solve this question is through trigonometry. :)
If you've learnt trigonometry, here are the basic formulas:
SOH CAH TOA (These formulas only apply for right-angled triangles)
Sin(angle) = (Side that is Opposite of angle)/(Hypotenuse)
Cos(angle) = (Side that is Adjacent of angle)/(Hypotenuse)
Tan(angle) = (Side that is Opposite of angle)/(Side that is Adjacent of angle)
In this case, we should use sine of b (the angle at the top of triangle as indicated in your question), where the opposite side is the base (I.e. 4 indicated in your question) and the hypotenuse is 9.
sin(b) = 4/9
{Make angle b the subject of the equation}
[tex]b = {sin}^{ - 1} ( \frac{4}{9} )[/tex]
{Type this into the calculator}
Angle b = 26.4° (rounded off to 1 decimal place)
Hope this helps! :)
