Ineed to find the dimensions but i have to sep up two equations to find out

Mp model with mathematics explain how the number of text messages dario sent and the cost form an arithmetic sequence. then write an expression to find dario's text messaging bill if he sends n text messages over 250.

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Each person in an it department of 25 people works on at least one of 3 major projects: system security, program implementation, platform development. 10 people work on system security, 13 work on program implementation, and 12 work on platform development. 4 people work on system security and platform development, 5 people work on program implementation and platform development, and 2 people work on all three projects. how many people work on system security and program implementation?