To figure out where each crosses the x and y axis simply plug in 0 for y and x respectively.
x intercept is where y = 0
y intercept is where x = 0
a) x intercept
ax + b(0) = c
c = ax
x = c/a so the x intercept is (c/a, 0)
y intercept
a(0) + by = c
0 + by = c
y = c/b, so the y intercept is (0,c/b)
Jus repeat the process for part b
b) x intercept
ax - 0 = c
x = c/a, so the x intercept is the same, (c/a , 0)
y intercept
0 - by = c
y = -c/b, so the y intercept is different from before (0, -c/b)