First off you have to understand that the axes are labeled whatever they happened to be labeled so you have to be careful using terms like x-intercept and y-intercept. In problem (b) the horizontal axis happens to be the t-axis and the vertical axis is the A axis.
Next we understand that the intercept of a given axis is then the value of the variable on the other axis is 0.
In problem (a) we find the horizontal intercept when the variable on the vertical axis, y, is 0. We find the vertical intercept when the variable on the horizontal axis, x, is 0.
In (a) y=3x + 24
The vertical intercept is easy, just let x=0 and evaluate y. Here we get
y=3*0 + 24 = 24
The horizontal intercept isn't quite as easy. Here we have to set y=0 and solve for x.
0 = 3x+24
-3x = 24
x = -8
So we see the vertical intercept is at (0,24), and the horizontal intercept is at (-8, 0)
I'm going to let you work problem (b), just remember
vertical intercept: variable on horizontal axis, here t, equals 0.
horizontal intercept: variable on vertical axis, here A, equals 0.