Not sure how to work this any help would be great

Simplify 15(x - 4)(x -1)-3(x + 4)(x - 1) with steps

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%)

Idon't get undoing if its pass adding and subtracting so can someone ? x-2 over 5 = 18