I need help with math it’s due Saturday!

The mass of riboflavin in one small serving (75 g) of raw almonds is 0.87 mg less
than the mass of riboflavin in 21 small servings of raw almonds. What is the mass
of riboflavin in one small serving of raw almonds?

a) Write an equation that models the situation.

b) Solve the equation using guess and check.

c) Solve the equation by isolating the variable