y = mx + c refers to the proportionality (maybe you have recognized that?). So everything what you have to do is to rearrange your equation T= 2pi sqrt((h^2+k^2)/gh) to get the proportionality of simple harmonic motion.
I definitely would
have a think about what's constant and what's going to be variable?
When it is asked for proportionality, the graph is linear. in y = mx + c, x is the variable which increase the left side of the equation, so y. m stands for the slope and c is the y-intercept. All what you need is to rearrange the equation that this linear property of proportionality comes out. To square on both sides is the beginning.
Then what do i do
T² = 4pi²(h² + k²)/gh (to summarize)
= 4pi²h² + 4pi²k²/gh
= 4pi²h + 4pi²k²/g (to factorize)
= 4pi²(h + k²)/g
For g = constant, the formular and the graph of it should be linear after y = mx + c.