Final answer:
The St. Lawrence River is 3058km long and the Mackenzie River is 4241km long.
Step-by-step explanation:
Let's represent the length of the St. Lawrence River as x. Since the Mackenzie River is 1183km longer, its length can be represented as x + 1183. The sum of the two lengths is given as 7299km, so we can set up the equation:
x + (x + 1183) = 7299
Combining like terms and solving for x:
2x + 1183 = 7299
2x = 6116
x = 3058
Therefore, the St. Lawrence River is 3058km long and the Mackenzie River is 4241km long.