I really need help on this question

Two times a number plus three times another number equals 4. three times the first number plus four times the other number is 7. find the numbers

Find the domain and range of the following function f(x) = 51x - 21+ 4

Astudent must have an average on five test that is greater than it equal to 80% but less than 90% to receive a final grade of b. devon's greades on the first four test were 78% 62% 91% and 80% what range if grades on the fifth test would give him a b in the course? ( assuming the highest grade is 100%)