Determine the equation of the line using the given slope and point by,
y - y1 = m(x - x1)
Substitute the values,
y - 6 = 5(x - 5)
Simplifying the equation,
y = 5x - 19
Substituting the given value of x given,
y = 5(21) - 19 = 86
Therefore, the answer is 86.