How can I argue that Disraeli's decision to buy shares in the Suez canal in 1887 can be explained by peripheral theories?
Peripheral theories of colonialism argue that forces on the periphery of the empire cause the imperial power to do things like expanding. You can argue that Disraeli's decision falls into this category.
Disraeli had a chance to buy the shares in the canal in 1875 because the Khedive of Egypt needed money. Disraeli's reason for buying the shares had to do with France and India and Australia. Britain valued the Suez Canal as an important part of its trade routes with India and Australia. Thus, the Suez can be seen as a place on the periphery of the British Empire. Although it was peripheral, it was still important and Disraeli bought the shares so France could not buy them and thus control this important part of the trade route.
In this way, Disraeli's decision was based on factors that had to do with the periphery of the British Empire, not with Britain itself or with anything having to do with Egypt.