Analyzing problems involving definite integrals

The water level under a bridge is changing at a rate of $r(t)=40\sin\left(\dfrac{\pi t}{6}\right)$r, left parenthesis, t, right parenthesis, equals, 40, sine, left parenthesis, start fraction, pi, t, divided by, 6, end fraction, right parenthesis centimeters per hour (where $t$t is the time in hours). At time $t=3$t, equals, 3, the water level is $91$91 centimeters. By how much does the water level change during the $4^{\text{th}}$4, start superscript, start text, t, h, end text, end superscript hour?