Motion with constant acceleration - AS Maths

A car starts from rest at time t= 0. It accelerates uniformly until it's speed reaches Vm/s it travels at constant speed for 12 seconds and then decelerates uniformly coming to rest at t=26. The total distance travelled by the car is 840 metres. Find the value of V.

Hi, please could someone help me with this equation, no matter which way I look at it, I cannot find enough values to input into a SUVAT equation.

Thank you

This is the velocity time graph I drew, from this I did:
840=(6+26)/2 * V (where V is the height/speed at that point)

840 = 16V
V=52.5

I would've thought this is the answer however it is 44.2 ms^-1.
I'm not sure if I have made a silly mistake or done the question completely incorrectly......

This is the velocity time graph I drew, from this I did:
840=(6+26)/2 * V (where V is the height/speed at that point)

840 = 16V
V=52.5

I would've thought this is the answer however it is 44.2 ms^-1.
I'm not sure if I have made a silly mistake or done the question completely incorrectly......

This is the velocity time graph I drew, from this I did:

On your graph, you have only drawn the line being at constant speed for 6 seconds (12-18) but the question says the car was at constant speed for 12 seconds